Is Apple eSim on Watch 3 / iPhone XS proprietary standard? Is it a proprietary standard? I know GSMA has a standard by the same name but if it is not proprietary, then why does Apple list all the supported carriers on their website? They wouldn't know what carriers do and don't support the standard in let's say Germany? 

Is there some way Apple has control over what carrier they will allow to be used on the phone / watch?
eSIM is an open JEDEC standard for the interface to the hardware component of a SIM card designed to replace the UICC (smart-card) interface that is in common usage, the third-generation Apple Watch just happened to be the first major device to use it commercially.  There is actually no special support required for the hardware itself.

What is required for carrier support though is the ability to remotely provision the SIM.  Because an eSIM is not removable from the device, it has to be provisioned (configured for authentication to the carrier) in the device.  This particular aspect is what Apple's list is about, some carriers support it, some don't.  It's also really easy for them to find out which of the major ones do (they can literally just ask them, because Apple really is that big that they can do stuff like that).