Maybe someone can explain why their SSDs, when not soldered down, are a proprietary form factor and can't be replaced with a standard M.2 device?

I believe they have a special kind of SSD, where the controller is locked into the motherboard. This is serial locked in some form.

(Correct me if I’m wrong, but this is how I understand it)

My understanding is that the controller is integrated into the SoC.

You can actually replace the storage on Apple desktops but it’s a bit of a pain and isn’t as cheap as m.2 2280 drives.

I don’t really know why Apple does this when their solution doesn’t seem to offer an obvious advantage. They aren’t faster than existing solutions or anything like that.

Vendor lock-in increases their profit margins. If people could purchase a 128GB Mac mini and install their own 2TB drives they wouldn't make as much money.

To gain the same effect as one that is soldered down, while giving themselves a simpler portfolio of assemblies and a cheaper FRU to swap in the event of failure.

So vendor locking to make more money. :)