After watchOS 7.3.3 update activity app and heart rate doesn’t work at all

The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

Another thing you can try if your Watch appears to get wonky, especially after a watchOS upgrade/update,

Try a simple hard reset of your Apple Watch by holding both the Crown and side button simultaneously until your Watch displays the Apple Logo then let go of the buttons.

At this point, I suggest you contact Apple Support.