Not just laptops but the used enterprise micro PCs from Dell, HP, and Lenovo. All the same small form factor with very low TDP You can have up to 32 or 64 GB RAMs depending on the CPU, dual or even triple disks if you want a NAS etc.

I have seen quite a few the size of a Mac Studio / Intel Nuc, what are the device called that are the size of a pi?

yeah, depends on what the used market looks like where you live. Here I see way more laptops for sale for cheap than those enterprise thin clients.

And the thin clients when they are for sale tend to have their SSDs ripped out by IT for data security, so then it's a hassle to go out and buy and extra SSD, compared to just buying a used laptop that already comes with display , keyboard, etc.