See the linked questions in the comments.
The exact transliteration scheme will depend on the package(s)/code being used.
For Velthuis transliteration and IAST transliteration etc under devanagari package, you may possibly look for the .map files on your system, and see the Unicode values, to-and-from (<>) - although that will be time-consuming to trace, glyph by glyph, codepoint by codepoint.
In the meantime, you can do your own transliteration method using expl3 replace function, if you have the fonts to display the deva (like Shobika, Noto Serif Devanagari, Nirmala UI, etc), and if you can run fontspec package to handle the fonts (run in xelatex; or in lualatex with Renderer=HarfBuzz font option).
Example:
a \devats{} can be defined:

The core command is:
\tl_replace_all:Nnn \l_myts_tl { #1 } { #2}
"replace_command token_list_variable {from_string} {to_string}"
You can change the transliteration scheme to use different names/characters in the "from_string" part, if you want (e.g., typing anu instead of anudatta), but remember the general rule when doing replace: that longest strings must be replaced before shorter ones. See the MWE.
MWE
\documentclass{article}
\usepackage{xcolor}
\usepackage{fontspec}
\usepackage{multicol}
\usepackage{graphicx}
\setmainfont{Noto Serif}
\newfontfamily\fdeva[Colour=blue,
Script=Devanagari,
Renderer=HarfBuzz]{Noto Serif Devanagari}
%=====
%------------------------------- print and run #1
\ExplSyntaxOn
\tl_new:N \l_my_tl
\tl_new:N \l_myts_tl
\NewDocumentCommand{\cdr}{ m }{%
\tl_set:Nn \l_my_tl { #1 }
{ \ttfamily\color{blue}
\detokenize{#1}
}
\enspace $\mapsto$ \enspace
\tl_use:N \l_my_tl
}%
%----- Typing shortcut
\newcommand\scdvts[2]{%
\tl_replace_all:Nnn \l_myts_tl { #1 } { #2}
}
%===
\newcommand\dodevatransts{%
%
\tl_replace_all:Nnn \l_myts_tl { // } { ሴ}
\tl_replace_all:Nnn \l_myts_tl { / } { }
%
\scdvts { dcandrabvirama } { ꣴ }
\scdvts { candrabvirama } { ꣳ }
\scdvts { candabspace } { ꣲ }
\scdvts { candrabinv } { ऀ }
\scdvts { cmavagraha } { ꣱ }
\scdvts { cbavagraha } { ꣷ }
\scdvts { anusvara } { ं }
\scdvts { ecandra- } { ऍ }
\scdvts { ocandra- } { ऑ }
\scdvts { avagraha } { ऽ }
\scdvts { anudatta } { ॒ }
\scdvts { eecandra } { ॕ }
\scdvts { pushpika } { ꣸ }
\scdvts { candrab } { ँ }
\scdvts { visarga } { ः }
\scdvts { ecandra } { ॅ }
\scdvts { ocandra } { ॉ }
\scdvts { acandra } { ॲ }
\scdvts { siddham } { ꣼ }
\scdvts { virama } { ् }
\scdvts { eprish } { ॎ }
\scdvts { udatta } { ॑ }
\scdvts { abbrev } { ॰ }
\scdvts { yheavy } { ॺ }
\scdvts { filler } { ꣹ }
\scdvts { jainom } { ꣽ }
\scdvts { nukta } { ़ }
\scdvts { grave } { ॓ }
\scdvts { acute } { ॔ }
\scdvts { caret } { ꣺ }
\scdvts { dddh } { ड़ }
\scdvts { ooe- } { ॴ }
\scdvts { uue- } { ॷ }
\scdvts { cmvi } { ꣰ }
\scdvts { head } { ꣻ }
\scdvts { aa- } { आ }
\scdvts { ii- } { ई }
\scdvts { uu- } { ऊ }
\scdvts { ai- } { ऐ }
\scdvts { au- } { औ }
\scdvts { tth } { ठ }
\scdvts { ddh } { ढ }
\scdvts { nnn } { ऩ }
\scdvts { lll } { ऴ }
\scdvts { ooe } { ऻ }
\scdvts { rr= } { ॄ }
\scdvts { uue } { ॗ }
\scdvts { khh } { ख़ }
\scdvts { ghh } { ग़ }
\scdvts { rr- } { ॠ }
\scdvts { ll- } { ॡ }
\scdvts { ll= } { ॣ }
\scdvts { dot } { ॱ }
\scdvts { oe- } { ॳ }
\scdvts { aw- } { ॵ }
\scdvts { ue- } { ॶ }
\scdvts { dda } { ॸ }
\scdvts { ddd } { ॾ }
\scdvts { cma } { ꣪ }
\scdvts { cmu } { ꣫ }
\scdvts { cmk } { ꣬ }
\scdvts { cmn } { ꣭ }
\scdvts { cmp } { ꣮ }
\scdvts { cmr } { ꣯ }
\scdvts { cb2 } { ꣵ }
\scdvts { cb3 } { ꣶ }
\scdvts { ay- } { ꣿ }
\scdvts { a= } { ऄ }
\scdvts { a- } { अ }
\scdvts { i- } { इ }
\scdvts { u- } { उ }
\scdvts { r- } { ऋ }
\scdvts { l- } { ऌ }
\scdvts { e= } { ऎ }
\scdvts { e- } { ए }
\scdvts { o= } { ऒ }
\scdvts { o- } { ओ }
\scdvts { kh } { ख }
\scdvts { gh } { घ }
\scdvts { ng } { ङ }
\scdvts { ch } { छ }
\scdvts { jh } { झ }
\scdvts { ny } { ञ }
\scdvts { tt } { ट }
\scdvts { dd } { ड }
\scdvts { nn } { ण }
\scdvts { th } { थ }
\scdvts { dh } { ध }
\scdvts { ph } { फ }
\scdvts { bh } { भ }
\scdvts { rr } { ऱ }
\scdvts { ll } { ळ }
\scdvts { sh } { श }
\scdvts { ss } { ष }
\scdvts { oe } { ऺ }
\scdvts { aa } { ा }
\scdvts { ii } { ी }
\scdvts { uu } { ू }
\scdvts { r= } { ृ }
\scdvts { e. } { ॆ }
\scdvts { ai } { ै }
\scdvts { o. } { ॊ }
\scdvts { au } { ौ }
\scdvts { aw } { ॏ }
\scdvts { om } { ॐ }
\scdvts { ue } { ॖ }
\scdvts { rh } { ढ़ }
\scdvts { yy } { य़ }
\scdvts { l= } { ॢ }
\scdvts { || } { ॥ }
\scdvts { zh } { ॹ }
\scdvts { gg } { ॻ }
\scdvts { jj } { ॼ }
\scdvts { gs } { ॽ }
\scdvts { bb } { ॿ }
\scdvts { c0 } { ꣠ }
\scdvts { c1 } { ꣡ }
\scdvts { c2 } { ꣢ }
\scdvts { c3 } { ꣣ }
\scdvts { c4 } { ꣤ }
\scdvts { c5 } { ꣥ }
\scdvts { c6 } { ꣦ }
\scdvts { c7 } { ꣧ }
\scdvts { c8 } { ꣨ }
\scdvts { c9 } { ꣩ }
\scdvts { ay } { ꣾ }
\scdvts { k } { क }
\scdvts { g } { ग }
\scdvts { c } { च }
\scdvts { j } { ज }
\scdvts { t } { त }
\scdvts { d } { द }
\scdvts { n } { न }
\scdvts { p } { प }
\scdvts { b } { ब }
\scdvts { m } { म }
\scdvts { y } { य }
\scdvts { r } { र }
\scdvts { l } { ल }
\scdvts { v } { व }
\scdvts { s } { स }
\scdvts { h } { ह }
\scdvts { i } { ि }
\scdvts { u } { ु }
\scdvts { e } { े }
\scdvts { o } { ो }
\scdvts { q } { क़ }
\scdvts { z } { ज़ }
\scdvts { f } { फ़ }
\scdvts { | } { । }
\scdvts { 0 } { ০ }
\scdvts { 1 } { ১ }
\scdvts { 2 } { ২ }
\scdvts { 3 } { ৩ }
\scdvts { 4 } { ৪ }
\scdvts { 5 } { ৫ }
\scdvts { 6 } { ৬ }
\scdvts { 7 } { ৭ }
\scdvts { 8 } { ৮ }
\scdvts { 9 } { ৯ }
%
\tl_replace_all:Nnn \l_myts_tl { ሴ} { / }
\tl_replace_all:Nnn \l_myts_tl { a } { }
}
%=====
\NewDocumentCommand { \devats } { m } { \devatransts {#1} }
\NewDocumentCommand { \devatransts } { m } {%
\tl_set:Nn \l_myts_tl { #1 }
\dodevatransts
\tl_use:N \l_myts_tl
}
%---- Environments
\NewDocumentEnvironment{devatranstse}{ O{\fdeva} +b }
{
#1
\tl_set:Nn \l_myts_tl { #2 }
\dodevatransts
\tl_use:N \l_myts_tl
}
{ }
\ExplSyntaxOff
\newcommand\dv[1]{\noindent\cdr{\devats{#1}}\par}
\begin{document}
\section{Sample}
\subsection{Typing Shortcuts}
{\fdeva
\begin{multicols}{2}
\dv{candrabinv}
\dv{candrab}
\dv{anusvara}
\dv{visarga}
\dv{a=}
\dv{a-}
\dv{aa-}
\dv{i-}
\dv{ii-}
\dv{u-}
\dv{uu-}
\dv{r-}
\dv{l-}
\dv{ecandra-}
\dv{e=}
\dv{e-}
\dv{ai-}
\dv{ocandra-}
\dv{o=}
\dv{o-}
\dv{au-}
\dv{k}
\dv{kh}
\dv{g}
\dv{gh}
\dv{ng}
\dv{c}
\dv{ch}
\dv{j}
\dv{jh}
\dv{ny}
\dv{tt}
\dv{tth}
\dv{dd}
\dv{ddh}
\dv{nn}
\dv{t}
\dv{th}
\dv{d}
\dv{dh}
\dv{n}
\dv{nnn}
\dv{p}
\dv{ph}
\dv{b}
\dv{bh}
\dv{m}
\dv{y}
\dv{r}
\dv{rr}
\dv{l}
\dv{ll}
\dv{lll}
\dv{v}
\dv{sh}
\dv{ss}
\dv{s}
\dv{h}
\dv{oe}
\dv{ooe}
\dv{nukta}
\dv{avagraha}
\dv{aa}
\dv{i}
\dv{ii}
\dv{u}
\dv{uu}
\dv{r=}
\dv{rr=}
\dv{ecandra}
\dv{e.}
\dv{e}
\dv{ai}
\dv{ocandra}
\dv{o.}
\dv{o}
\dv{au}
\dv{virama}
\dv{eprish}
\dv{aw}
\dv{om}
\dv{udatta}
\dv{anudatta}
\dv{grave}
\dv{acute}
\dv{eecandra}
\dv{ue}
\dv{uue}
\dv{q}
\dv{khh}
\dv{ghh}
\dv{z}
\dv{dddh}
\dv{rh}
\dv{f}
\dv{yy}
\dv{rr-}
\dv{ll-}
\dv{l=}
\dv{ll=}
\dv{|}
\dv{||}
\dv{0}
\dv{1}
\dv{2}
\dv{3}
\dv{4}
\dv{5}
\dv{6}
\dv{7}
\dv{8}
\dv{9}
\dv{abbrev}
\dv{dot}
\dv{acandra}
\dv{oe-}
\dv{ooe-}
\dv{aw-}
\dv{ue-}
\dv{uue-}
\dv{dda}
\dv{zh}
\dv{yheavy}
\dv{gg}
\dv{jj}
\dv{gs}
\dv{ddd}
\dv{bb}
\dv{c0}
\dv{c1}
\dv{c2}
\dv{c3}
\dv{c4}
\dv{c5}
\dv{c6}
\dv{c7}
\dv{c8}
\dv{c9}
\dv{cma}
\dv{cmu}
\dv{cmk}
\dv{cmn}
\dv{cmp}
\dv{cmr}
\dv{cmvi}
\dv{cmavagraha}
\dv{candabspace}
\dv{candrabvirama}
\dv{dcandrabvirama}
\dv{cb2}
\dv{cb3}
\dv{cbavagraha}
\dv{pushpika}
\dv{filler}
\dv{caret}
\dv{head}
\dv{siddham}
\dv{jainom}
\dv{ay}
\dv{ay-}
\end{multicols}
}
\section{Environments}
\noindent devatranstse:
\begin{devatranstse}
klmn
/k/l/m/n
\end{devatranstse}
\section{Usage}
{\fdeva
\Large
{\fdeva \devats{devanagari}}
{\fdeva \devats{kulviramabananas}}
\begin{multicols}{2}
{\fdeva
\dv{m}
\dv{ma}
\dv{maa}
\dv{mi}
\dv{mii}
\dv{mu}
\dv{muu}
\dv{mr=}
\dv{mrr=}
\dv{ml=}
\dv{mll=}
\dv{me.}
\dv{me}
\dv{mai}
\dv{mo.}
\dv{mo}
\dv{mau}
\dv{moe}
\dv{mooe}
\dv{macandra}
\dv{mecandra}
\dv{meecandra}
\dv{mocandra}
\dv{meprish}
\dv{maw}
\dv{mue}
\dv{muue}
\dv{mudatta}
\dv{manudatta}
\dv{mgrave}
\dv{macute}
\dv{mnukta}
\dv{mdot}
\dv{mc0/mc1/mc2}
\dv{mc3/mc4}
\dv{mc5/mc6}
\dv{mc7/mc8/mc9}
\dv{mcma/mcmu}
\dv{mcmk/mcmn}
\dv{mcmp/mcmr}
\dv{mcmvi}
\dv{mcmavagraha}
}
\end{multicols}
\end{document}
devanagaripackage. There is a manual for this package on CTAN (https://ctan.org/pkg/devanagari) which also includes a lot of information about the required input and the respective output of the different characters. – Jasper Habicht Jul 28 '21 at 09:07dnmethod from 1991. – Cicada Aug 01 '21 at 06:17