KNX smart home intercom recommendation - single-family house

Yes, I initially bought the set with one indoor station. Additional ones can be retrofitted for 120-150 EUR.
So far, unfortunately no practical experience, hopefully more soon.

However, I am also an iOS user and am curious about the experiences with iPhone/iPad in this context.
At the moment, I unfortunately can't say much..., but in principle, everything should work.

And which ones?

It will probably come down to one app for the house and one for the door. The door opener must then be operated from the doorbell app (as described above with the relays, pay attention to the wiring beforehand!).

Yes, with the Doorbird app and via the Doorbird cloud. Doorbird cannot do Homekit.

Yes, with the Doorbird app and via the Doorbird cloud. Doorbird does not support Homekit.