MacBook Air (M1, 2020)
I haven't had enough time to really explore all over the place, but one thing that I was eager to check into was the new built-in 2-factor authentication feature, and I can report that it works flawlessly. I was using OTP Auth before, and thought that was an excellent Mac/iOS/iCloud combo solution, but now I don't even have to go and GET codes to enter, it all happens automagically. It's fantastic! In order to migrate from OTP Auth to the built-in system, I had to manually view each saved account in OTP Auth and display the QR code, make a screenshot of it, and then either (1) open that screenshot in Safari in order to right-click on it and choose "Setup Verification Code", which would take you immediately to Safari's password preferences section (you might need to enter your password first) and display whichever saved password was the correct one (usually there would only be one, but if I had this setup for, say, multiple Google accounts, I had to pick the proper one out of several shown), or (2) create screenshots for each QR code from OTP Auth all back to back, naming each one descriptively, and then using my iPhone's QR code scanner to actually complete the setup process on the phone rather than doing it all entirely on the Mac. This had the side effect of causing me to reorganize and streamline the folder where I saved all my account 2-factor/backup codes/recovery keys files, and gave me the entirely new idea of creating an encrypted disk image to move all of this into for extra security.
In order to test the 2-factor authentication for each and every site, I would have to remove each one's cookies to purge not only my login info but also my "do not ask again on this browser" preference. I logged out and back in again on 4 different sites and it worked flawlessly each time, so I'm taking it on faith that the rest will also work flawlessly. I will keep OTP Auth around for now just in case I goofed up somewhere.
-Pete