It’s not actually serial locked, you can replace SSDs by desoldering and resoldering a NAND chip. But basically their SSD controller is integrated into the M-series chips (mostly because cheaper, less connectors to manufacture). But it’s a very specialized process

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.

Note that before the Mac mini socketed SSDs, Retina MacBook Pros had socketed SSDs as well. They were not standard M.2 either though, you had to get specific aftermarket parts from the likes of OWC