What's the difference between different LaTeX versions like TeX Live, KaTex Mac TeX etc? There are so many of them but I don't really see a difference.
TeX Live is an official distribution of essentially the entire TeX/LaTeX ecosystem, including all the processing engines and appropriately available LaTeX packages. TeX Live is available for multiple platforms.
Mac TeX is the same as TeX Live but is specifically designed to be installed on Mac computers. It also contains several Mac GUI apps (e.g. TeXShop) for editing documents. These GUI apps are entirely optional and are available separately.
KaTeX is not LaTeX at all. It is a mathematical typesetting library designed for use on websites. It, like MathJax, uses a LaTeX-like syntax for its commands and produces output that looks very much like that of LaTeX, but neither KaTeX nor MathJax are actual LaTeX. Neither can make use of external packages. Questions about KaTeX and MathJax are usually off topic here since they do not rely on TeX or LaTeX.
I can elaborate further if desired.