Does this require Google Play Services on Android? If so that is a shame. There are other FIDO SDKs other than Google's which would be nice for us degoogled users. Edit: it requires play services. Sigh.
So if I add a security key on my phone, I'll have to use it on the web too? Or would I need to make sure I have multiple keys so I can log in with either one?
Not sure I understand the question. A key can be used on Web as well as on the mobile apps now, instead of TOTP (which however cannot be yet disabled). It is generally advised to have a multiple key pair, incase one is lost / defect.
I thought I read that some phones can be the security key, hence my confusion as to how would that work
Proton
Empowering you to choose a better internet where privacy is the default. Protect yourself online with Proton Mail, Proton VPN, Proton Calendar, Proton Drive. Proton Pass and SimpleLogin.
Proton Mail is the world's largest secure email provider. Swiss, end-to-end encrypted, private, and free.
Proton VPN is the world’s only open-source, publicly audited, unlimited and free VPN. Swiss-based, no-ads, and no-logs.
Proton Calendar is the world's first end-to-end encrypted calendar that allows you to keep your life private.
Proton Drive is a free end-to-end encrypted cloud storage that allows you to securely backup and share your files. It's open source, publicly audited, and Swiss-based.
Proton Pass Proton Pass is a free and open-source password manager which brings a higher level of security with rigorous end-to-end encryption of all data (including usernames, URLs, notes, and more) and email alias support.
SimpleLogin lets you send and receive emails anonymously via easily-generated unique email aliases.