How you set up and use passkeys?

Here's one for the Mac. You can find the others by googling passkeys with iPhone and iPads.

Sign in to an account on your Mac with a passkey - Apple Support

When you initially sign into a web site, for example, you are asked for an ID and a password. These must then be entered. You will then be asked if you wish to save them. If you select YES, the information is then saved into the Mac's Keychain app for future automated entry. In subsequent accesses to the web site you will be asked if you want to use the saved password. Then you again respond with yes. It's all very easy and works well.

DO keep in mind that passkeys depend on development of EACH individual web site requiring a log in.

It’s not a “blanket solution” which works everywhere; and where it does work today, it’s often “baby steps.”

The industry transition away from username & password is a HUGE cultural change which will likely be an prolonged “work in progress.”