How do I choose a Mariadb version to install, and what is the difference between mariadb.com and .org?
On downloads, I'd go with the MariaDB.org repository management tool. There is only one 20.04 focal release for the 10.5 Beta branch at the moment (so not recommended yet for production use). The reason is that the releases include what distro builders where available at the time of release so as time goes on there will be a focal repository for every major release.

Until that happens, focal includes a 10.3.22 package version which is a good starting point.

On a normal fresh install I'd pick the latest GA stable (10.4) however since there isn't easy packages for focal yet, stick with the 10.3.22 distro package.

MariaDB.com releases are tarball only installs making upgrading harder.

On MySQL vs MariaDB, there is a large (and dry reading) compatibility page describing differences.

As its a green fields install there isn't a lot to worry about with compatibility. The main thing to look at is if MySQL/MariaDB has the features you need.