The .bib file format is used to describe bibliographical references. When used together with LaTeX and BibTeX, it generates a file with the .bbl extension, which is really just a tex file with a different extension containing your references formatted according to the bibliography style chosen in your .tex file through the \bibligraphystyle{} command. LaTeX then automatically inserts the contents of the bbl file in the exact place you call the \bibliography{} command in your document.
Usually, when working with TeX documents, journal and conference editors don't want to go to the trouble of using BibTeX to compile your paper; they prefer to defer the responsibility of sending the bibliography correctly formatted to the paper author (you). So, they tell you to manually do what pdflatex and bibtex automate for you: to get the contents of the bbl file and put them where you would call the \bibliography{} command. This way, you only have to send them a single tex file, instead of the tex file along with the bib file.
.bblfiles is for marking up the references (in.bibfile) while the file.bibcontains just the references? – Avv Jan 26 '23 at 14:58