Thank you Joseph and Frank for your help and explanations which
were really useful to me.

Inspired by your replies, I have found many useful links on SE,
packsges in TeXLive, and references to read about expl3 and
LaTeX3, including:

https://tex.stackexchange.com/search?q=expl3
https://tex.stackexchange.com/search?q=\ExplSyntaxOn
https://tex.stackexchange.com/questions/tagged/latex3

https://www.texdev.net/2014/02/02/programming-latex3-a-summary-so-far/

fontspec, siunitx, spath3, unicode-math

packages in l3packages directory
packages in l3experimental directory

Thanks,
Tisha