activity Mountainbike

but I can still change the activity cycling to mountain bike, e-mountain bike or road bike afterwards....

sorry for my english, but i use translator google

Check the app store, there are apps where you can start recording as mountain biking or other activities that Garmin doesn't support on the watch itself.

I do not understand you, how can I download an app on the watch and then start mountain bike and is it available in the Garmin Connect?

You can download Garmin Connect IQ where you can do a search for mountain biking and download whichever app you like which you will then use to start the activity on your watch and the activity will be saved to Garmin Connect.