For example:
\begin{longtable} { |L{2.75cm}|c|c|c|c| }
If the centered columns are already fixed width, is there anything I can write (where 2.75cm is written) to make the first column have a width such that the table occupies the full width of the page within its margins?
MWE with xltabular instead of longtable (doesn't compile):
\documentclass[12pt, a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{float}
\setlength{\parskip}{1em}
\usepackage[a4paper, total={6in, 8in}]{geometry}
\graphicspath{ {images/} }
\newcommand{\degree}{\ensuremath{^\circ}}
\newcommand{\quotes}[1]{``#1''}
\setlength\parindent{0pt}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{imakeidx}
\makeindex[columns=3, title=Alphabetical Index, intoc]
\usepackage[spanish,es-nodecimaldot]{babel}
\usepackage[sorting=none]{biblatex}
\addbibresource{Bibfile.bib}
\DeclareMathOperator\grad{grad}
\DeclareMathOperator\prima{'}
\DeclareMathOperator\sen{sen}
\usepackage{array}
\usepackage{tabularx}
\usepackage{lscape}
\usepackage{caption}
\captionsetup[table]{position=bottom}
\usepackage{booktabs}
\usepackage{mathptmx}
\usepackage[per-mode=symbol]{siunitx}
\usepackage{bm}
\usepackage{amsmath}
\newcolumntype{L}[1]{>{\raggedright\arraybackslash}p{#1}}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\usepackage{longtable}
\interfootnotelinepenalty=10000
\usepackage[spanish]{minitoc}
\usepackage{hyperref}
\usepackage{siunitx}
\sisetup{group-digits=false}
\usepackage{mathtools, nccmath}
\usepackage[raggedright]{titlesec}
\mathchardef\mhyphen="2D
\usepackage{xltabular}
\title{Tabla 1}
\author{-}
\date{September 2021}
\begin{document}
\maketitle
\section{Introduction}
\begin{xltabular}{ |X|c|c|c|c| }
\caption[Datos del tren de aterrizaje de aviones semejantes.]{\label{tren}}
\endlastfoot
\hline
\multicolumn{5}{|c|}{\textbf{Tren de Aterrizaje}} \
\hline
& Proyecto & \vtop{\hbox{\strut \makebox[2.5cm][c]{A319-100}\par }\hbox{\strut \makebox[2.5cm][c]{\cite{A319APM,JanesA319,A319TC,EurocontrolA319}}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{B737-700}\par }\hbox{\strut \makebox[2.5cm][c]{\cite{B737APM,JanesB737,B737TC,EurocontrolB737}}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{A220-300}\par }\hbox{\strut \makebox[2.5cm][c]{\cite{A220APM,JanesA220,A220TC,EurocontrolA220}}\par}} \
\hline
Tipo & \vtop{\hbox{\strut \makebox[1cm][c]{Triciclo}\par }\hbox{\strut \makebox[1cm][c]{retráctil}\par}} & \vtop{\hbox{\strut \makebox[1cm][c]{Triciclo}\par }\hbox{\strut \makebox[1cm][c]{retráctil}\par}} & \vtop{\hbox{\strut \makebox[1cm][c]{Triciclo}\par }\hbox{\strut \makebox[1cm][c]{retráctil}\par}} & \vtop{\hbox{\strut \makebox[1cm][c]{Triciclo}\par }\hbox{\strut \makebox[1cm][c]{retráctil}\par}} \
\hline
Vía\footnote{La vía $T$ del tren de aterrizaje es la distancia entre el centro de las huellas de los trenes principales. Si son más de dos, tomamos los más alejados del plano de simetría del avión.} $T$ (m) & 6.9338 & 7.59 & 5.72 & 6.73 \
\hline
Batalla\footnote{La batalla $B$ del tren de aterrizaje es la distancia longitdinal entre el centro de la huella del tren auxiliar y los centros de las huellas de los trenes principales. Si hay más de dos tomamos los de mayor distancia longitudinal respecto al tren auxiliar.} $B$ (m) & 12.7734 & 11.039 & 12.6 & 14.63 \
\hline
Relación $T/B$ & 0.5428 & 0.6869 & 0.4540 & 0.4600 \
\hline
Relación $T/b$ & 0.1995 & 0.2226 & 0.1667 & $\sim$ 0.2093 \
\hline
Relación $B/{l_f}$ & 0.3653 & 0.3262 & 0.3915 & 0.3781 \
\hline
Número de neumáticos por pata\footnote{Para los datos de las patas del tren de aterrizaje, usamos el subíndice $p$ para referirnos a las patas del tren principal, y el subíndice $a$ para referirnos al tren auxiliar.} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$N_p = 2$}\par}\hbox{\strut \makebox[2.5cm][c]{$N_a = 2$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$N_p = 2$}\par}\hbox{\strut \makebox[2.5cm][c]{$N_a = 2$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$N_p = 2$}\par}\hbox{\strut \makebox[2.5cm][c]{$N_a = 2$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$N_p = 2$}\par}\hbox{\strut \makebox[2.5cm][c]{$N_a = 2$}\par}} \
\hline
Denominación del neumático & \vtop{\hbox{\strut \makebox[3.25cm][c]{Pr: H42x16-19}\par}\hbox{\strut \makebox[3.25cm][c]{Aux: 25.75x6.75-14}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{Pr: 46x16-20}\par}\hbox{\strut \makebox[2.5cm][c]{Aux: 30x8.8-15}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{Pr: 43.5x16-21}\par}\hbox{\strut \makebox[2.5cm][c]{Aux: 27x7.7-15}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{Pr: 42x15-21}\par}\hbox{\strut \makebox[2.5cm][c]{Aux: 27x8.5-12}\par}} \
\hline
Diámetro (m) & \vtop{\hbox{\strut \makebox[2.5cm][c]{$D_p = 1.0668$}\par}\hbox{\strut \makebox[2.5cm][c]{$D_a = 0.6541$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$D_p = 1.1784$}\par}\hbox{\strut \makebox[2.5cm][c]{$D_a = 0.762$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$D_p = 1.1049$}\par}\hbox{\strut \makebox[2.5cm][c]{$D_a = 0.6858$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$D_p = 1.0668$}\par}\hbox{\strut \makebox[2.5cm][c]{$D_a = 0.6858$}\par}} \
\hline
Ancho (m) & \vtop{\hbox{\strut \makebox[2.5cm][c]{$b_p = 0.4064$}\par}\hbox{\strut \makebox[2.5cm][c]{$b_a = 0.1715$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$b_p = 0.4064$}\par}\hbox{\strut \makebox[2.5cm][c]{$b_a = 0.2235$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$b_p = 0.4064$}\par}\hbox{\strut \makebox[2.5cm][c]{$b_a = 0.1956$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$b_p = 0.381$}\par}\hbox{\strut \makebox[2.5cm][c]{$b_a = 0.2159$}\par}} \
\hline
Presión de inflado (MPa) & \vtop{\hbox{\strut \makebox[2.5cm][c]{$p_p = 1.31$}\par}\hbox{\strut \makebox[2.5cm][c]{$p_a = 1.372$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$p_p = 1.19$}\par}\hbox{\strut \makebox[2.5cm][c]{$p_a = 1.14$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$p_p =$ 1.36 - 1.41}\par}\hbox{\strut \makebox[2.5cm][c]{$p_a = 1.42$}\par}} & \vtop{\hbox{\strut \makebox[2.5cm][c]{$p_p = 1.47$}\par}\hbox{\strut \makebox[2.5cm][c]{$p_a = 1$}\par}} \
\hline
\end{xltabular}
\printbibliography
\end{document}
longtableandtabularx. This is achieved usingltablexand similar packages. – Werner Sep 13 '21 at 19:36xltabularenvironment, from the homonymous package, which brings the functionalities (and syntax) oflongtabletotabularxand replace{|L|c|c|c|c|}with{|X|c|c|c|c|}. – Bernard Sep 13 '21 at 19:38\begin{xltabular}{\textwidth}{|X|c|c|c|c| }. – Bernard Sep 13 '21 at 19:54siunitxandinputencfor example. The latter of which is not required in up-to-date systems anyways. Loading packages multiple times can also always facilitate errors and option clashes and should be avoided. – Markus G. Sep 13 '21 at 19:56\textwidth, as I wrote, if its really between the text margins. – Bernard Sep 13 '21 at 19:58