Indoor bike - Zwift compatible

Hey guys,
New here - I searched the forum but haven’t find the answer:
my question is what makes the bike “smart”, so it can be used with Zwift. Will any indoor bike with Bluetooth 4.0 work or it has to have some other features.

From what I understand the “smart” bike means that it can adjust resistance when you are for example going up the hill. The indoor bike with JUST bluetooth will let me play, but the app will not be able to communicate back to the bike and change the resistance. Is that right ?