Does there exist documentation for programmers regarding
Latex3 ? With details about
\use, and so on ?
As with all texlive packages, you have documentation installed on your system.
texdoc expl3 will give a top level view
texdoc interface3 is a reference manual of every defined function in the L3 kernel.
In general the LaTeX project website has links to documentation and announcements of new features
Yes. See the relevant PDF documents listed here: https://www.ctan.org/pkg/l3kernel.