Most modern operating systems randomize the MAC.
[citation needed]
having the option to randomize the MAC is not the same as actually doing that. There are also a few downsides to random MACs, like captive portals not remembering you on public WiFis.
You mean server? If so, the server is also open source
That is what i meant. It is OSS but not FOSS because you need a key to start it.
Bitwarden has a FOSS client app and FOSS server apps exist (though the default service is not FOSS).
Syncing 2FA keys brings the danger with it that you accidentally sync the key to the device that is used for the first factor thus making it not 2FA anymore.
You are technically correct in saying that it’s not a registration. Instead it’s a sponsoring: https://en.wikipedia.org/wiki/Sponsored_top-level_domain