Here’s the manual for a Fenix 6 - tip, search for pairing.
EDIT: if you want your watch to record your Zwift events, then you can pair everything up but speed wont be accurate. I dual record but dont care for lack of speed on Garmin Connect.
You can also broadcast your HR to Zwift from your Fenix. Lookup up Broadcast heart rate.
Yes, Kickrs pair just fine with Fenix watches.