There are no slots, it's all soldered directly to the motherboard. Even in the Mac Studio, which does use modular SSDs, they're proprietary modules rather than anything you can easily swap out yourself.

So not even milling off the ssd works now?

Fortunately I don't really see the point of using a mac mini, so this doesn't bother me too much, but... it's poor taste. You're holding it wrong was not cool the first time.

You can mill them off and replace them with supported nands. People have videos on YouTube but it’s not very accessible to do.

The issue is that Apple moved the storage controllers into their SOC. So they use raw nand chips, and you need to use ones that the SOC supports.