Uh... don't. Even! GET. MEEEE. STAAAAARTED!!!
Seriously, while I could even understand that there might be reasons for Apple to have different frames to connect the SSDs to their mainboards, while I could even force myself into accepting some reasons (even if their BS) why SSDs have to be soldered in (WHAT.THE... okok, I won't go into that), something that is just not acceptable is their upgrade prices. To take you from 512GB to 2TB internal SSD space, Apple is asking you SIX-HUNDRED-AND-NINETY Euros!!!
What the heck? Did they, unknown to the rest of the world, all of a sudden reinvent the SSD?
No. From all that is know, the ones Apple is using are of pretty decent quality but nothing outstanding. Possibly comparable to a Samsung 990 Pro M2. They're somewhat less than €200. So, where's those additional €500 coming from? Well, we don't know. But I know for sure where they're going to. Straight into Apple's ever so greedy maw.
My 2008 Macbook has 2TB worth of SSD space in it (on 2 1TB drives - something possible back in the day, another thing I defenitely miss) already. Since several years even, and I did't have to rob a bank. They're Samsung EVOs and were around 150 each back then when I bought the second one.