Are there any? I could find proprietary apps doing it from Google’s PlayStore but I really rather use a FOSS alternative if there are any. I tried searching on F-Droid and I couldn’t find any.
TeX is a typesetting language. Instead of visually formatting your text, you enter your manuscript text intertwined with TeX commands in a plain text file. You then run TeX to produce formatted output, such as a PDF file. Thus, in contrast to standard word processors, your document is a separate file that does not pretend to be a representation of the final typeset output, and so can be easily edited and manipulated.
More info on LaTeX