> If I accidentally set up a passkey on my phone (let’s say I use Safari one day instead of my go-to, Brave), can I still log in without that passkey on other devices?

N=1 and I'm sure I'm holding it wrong, but I can only log in to ADP to request PTO from my personal laptop because I set up an iCloud passkey, work laptop does not allow access to iCloud keychain, and you can't request PTO from mobile.

Although that's more a failure of your workplace's security policy than of the Passkey itself. It makes sense that the passkey doesn't work if you can't access the place the passkey is stored.