They're essentially certificates, so most implementations will only store them on-device, and most implementations I've seen seem to favor the phone as the device you use.

It really depends on the platform - but in short you'll either need a phone, or be locked into an ecosystem (browser, OS, etc) making using them on multiple devices & browsers difficult or impossible. A password manager supporting passkeys makes this easy as you can 1-click generate a passkey, and 1-click sign-in to services from any device or browser.