As per title, I have seen plenty of vids with certain default planes showing passengers inside. I am not talking about the passenger feature of GSX Pro. And I am also not interested in the passengers shown in career mode. How do I know if a plane supports depiction of passengers? Is there any list? Or do I see it in the selection menue? I guess only "true" MSFS2024 default planes will show them, correct? But as those are not marked in the selection menue...

Seriously no one in here knows this? Surprising...

I remember my very first flight in MSFS2024, when I flew the Caravan skydiving variant. The cabin was filled with skydivers, ready to jump with helmets and goggles. Since then, I've flown quite a few of the default aircraft but never seen any passengers. In theory, I would guess that loading all the default aircraft with passengers should be possible. 

IIRC you have to use the default weight and balance menu, not the EFB/tablets. This should then render in passenger into models that support it.

In the main menu:
click on the plane upper left.
click on Configure lower left.
click on Fuel and Payload
click on Manage Mass in EFB.
Now you see the screen where you can add passengers.

Or, also in the main menu:
click on EFB lower right
click on the aircraft icon.
click on Flight performances.
click on Mass and balance. 

The EFB can of course also be accessed from within the aircraft.

All three methods work for me