There will be a compartment the SSD will be saddled into inside of the console to which you will have access to from the outside. Every PlayStation console except for the original PlayStation adopts this method. An "in addition to" would simply make it proprietary, which are generally more expensive and limited.
I think i said that wrong. It won't be like a memory card that sticks out of the console.
I think it may just very well be expandable, instead of replaceable... which IMO is a better option. I don’t want to swap out the OEM ssd for another one, I would prefer to just add to it instead.
The Xbox one does suck that it’s a third party proprietary piece but if it works with no issue I see nothing wrong with that, it will add need storage and it’s placement has to do with cooling/airflow. Hopefully other companies hop on and make their own. Dude it’d be dope if I could use one of my 1tb micro sd cards with an adapter for it