The Garmin 935 is designed for triathletes, but has the same range of functions as the Fenix. The difference is that it is made of plastic and is lighter (see the "dcrainmaker" test report). I am thrilled with the watch. By the way, it also has a barometer and the sleep analysis is pretty accurate for me. I can only recommend the sports watch.
Yes, you can also track MTB, hiking, skiing and much more. With Move IQ, activities such as cycling are automatically recognised.

Fenix 5s Plus also has an altimeter.

Forerunner 935 is a multisport watch. I am satisfied with it. The heart rate measurement seems very accurate. The battery lasts a long time. GPS reception is tip-top. VO2max is calculated when jogging. In my opinion, this is the sport that gets the most out of this watch. The sleep phase measurement is not very accurate.

The Fenix 5s is also a multisport watch but what are the effective differences? Can it also be used for mountain biking, hiking, activity tracking etc.?