what exercise bike dose apple fitness+ use

There are bikes, which can sync with an Apple Watch, but that's a feature which you'd need to check out from any exercise bike manufacturer you may be interested in. Apple itself does use any specific exercise bike. I'm pretty sure Peloton, for example syncs with Apple Watch.

That said, you can use your Apple Watch to track your fitness on an any exercise bike, by choosing the right workout from workout types on your watch. This might help you --> Workout types on Apple Watch - Apple Support