Connect IQ updates on different Apps, Datafields without a change of the developer and same version number as before?

Yes - it's auto  migration.  It happens every once in a while.

One of my apps was auto-migrated but I didn't get a mail from Garmin.
https://apps.garmin.com/en-US/apps/cbc703ff-36cf-46bf-8cef-af71a0902688

How do I know what has been changed, if new products have been added I think I should know that so can test it myself in the simulator.
If make a new version but don't include the new devices in the manifest the users won't get the update.

I should Garmin should send a message to the developer.

I think Jim answered that here:

https://forums.garmin.com/developer/connect-iq/f/discussion/223118/large-amount-of-app-downloads-on-tuesday

jim_m_58 said:

There's what's called "auto migration", where in the store, a new device can be added with no action by you.  This is often a new APAC version of a device.  That means anyone that's using your app get's the "new" version" The chart will show a spike but your total downloads wont really change.

Essentially, your app is identical, but more devices can use it.

They are identical just with a new device added.

There are cases where you might want to turn off auto migration, where a device might be similar but not similar enough for you app.  I've run into where something requires a baro altimeter, but a "similar for auto-migration" device might not have a baro for example.