Companies and providers (like banks) have to support it, but use is voluntary.
Check out the spec and legal framework, it actually makes sense and is open to different implementations, though you might need to certify it.
Kinda like the discrimination DB does for people using paper tickets vs those using the DB Navigator app.
As a separate device, it should be offline always IMO, and perhaps the size of a passkey. Or one of those banking devices with a display that show an authenticated text saying what you are confirming.
....wow, that would be reinventing the existing model of the leading ID cards....
Crazy if you think about it :)
Private smartphones are excluded already.