If you save it via Passwords app, it'll be iOS / macOS mainly, but you can unlock with any Apple device that supports Passkey / Passwords app (so likely modern + reasonably updated) the easiest. If you want it to work "everywhere" then you CAN use your iOS / MacOS Passkey, it will show you a QR code, some places poorly support this, I believe both devices need bluetooth, and then it will authenticate it.

Linux is the only oddball here, I had issues getting this flow to work.

If the UX for Passkey improves, I will go all-in on it, I'm at the point I'd love to just completely block passwords from accessing my account, unless I explicitly enable it temporarily by logging on via passkey, I wish some sites would let me lock my account to this level, it would be better. Passwords feel like they just wind up all over the web.

Weirdly enough you can store multiple passkeys for a given domain, which can get confusing in some cases if they dont have normal names tied to them.

Edit: Originally I thought Passwords from Apple was iOS / macOS only, but its not! So I have been editing my original message, sorry for the confusion, I had forgotten that I can login on Windows with my Passkeys from Apple's ecosystem.

As another poster noted, you can transfer them out of Apple's ecosystem too!