Apple - TeX on Arm Infrastructure

As it says on https://www.tug.org/mactex/aboutarm.html

After installing MacTeX-2020-Universal, all front ends and the command line programs will automatically use Arm binaries on Arm machines and Intel binaries on Intel machines.

so if you start from scratch and want a working TeX installation which runs natively just install the Universal package and you are all set. The remarks about the 2021 version providing full ARM support most probably refers to extended testing and maybe adapting some programs to make better use of ARM.

As for upgrading to 2021 next year, this will work the same way as before: The 2021 package will install into its own directory within /usr/local/texlive and you can then switch to the new version with the TeX Live Utility. Any user-installed styles and fonts are in /usr/local/texlive/texmf-local so they will remain available.

PS: Using Homebrew has the advantage of updating TeXLive automatically whenever you update your Homebrew installation. OTOH Homebrew currently is still transitioning to ARM so IMHO it's better to install MacTeX directly and run tlmgr update --self; tlmgr update --all every few weeks to keep it updated.

Regarding your question B:

MacTeX is released in a new, official version once a year. I.e. MacTeX 2019, MacTeX 2020, etc.

In between these releases, development occurs - and various "interim" versions can be made at any time. The Universal package with ARM and Intel code is such an "interim" version.

I.e. if you want the official version, which will most probably be the version that works best, fewest bugs and problems - you want the MacTeX 2020 version that is not universal.

If you want to something that runs faster and you're OK with getting an "in-between" version that hasn't been tested as well as the ordinary versions, then get the Universal version.

In any case upgrading to the new version next year is easy, and does not require you to start from scratch.