Can ONLY install Win10 on eMMC, not SSD

So, a couple of days ago I posted this: https://www.reddit.com/r/techsupport/comments/vlb6mc/impossible_to_install_windows10_on_m2_ssd/idu59a2/?context=3

TL;DR When I try a clean install of Win10 on a new M.2 SSD in a cheap Laptop, the installation fails at first reboot and leads to startup.nsh / EFI-shell. No Windows Boot Manager in BIOS. SSD shows up correctly in BIOS. Tried a gazillion solutions, nothing works. Only way to install Win10 on this laptop is to use the soldered 32gb eMMC. Works perfectly fine, but it's not what I want.

Can anyone please explain to me, old-school nerd girl, what could POSSIBLY be the reason that Windows can't finalize the installation on the SSD? But it works immediately if I chose the eMMC?

Maybe if I figure that out, I might find a solution to the problem...