The micro/tiny form factor PCs generally only have space for a 2.5" drive, while the SFF/desktop size PCs can fit at least one 3.5" drive.

Dell, 8th Gen Intel with one 3.5" drive bay: https://www.dell.com/support/manuals/en-us/optiplex-7060-sff/opti_7060_sff_setup_specs_manual/storage?guid=guid-4eab9a0b-c403-4bcd-b692-9419bddfed24&lang=en-us

Lenovo, should have one 3.5" drive bay: https://www.lenovo.com/us/en/p/desktops/thinkcentre/m-series-sff/thinkcentre-m920s/11tc1mdm92s?orgRef=https%253A%252F%252Fwww.google.com%252F&srsltid=AfmBOorJ_EUW-QYIIIGy3i7ZJbitPq8UWoxdoQxtM_BNabrKcNojn7WL#tech_specs

Thing is - “mini” PCs old enough to have 3.5 slots are probably way too old to have decent CPUs.

just want some old junker (6/7/8th gen Intel)

You probably have to go back further than that for a 3.5" sff pc. Look on woot though, they have such refurbs all the time. Or scrounge a mini tower.

If you can find them the "mini" format Elitedeak has a 3.5" internal bay along with an optical, for what it's worth. It's not as small as a "micro" but it's smaller than a tower, and at my hearby Uni, they go for the same (cheap) prices.

Why 3.5" drive? (Just curious).

I've found prices aren't necessarily any better at that size.

There's a few companies selling a very plasticky mini-pc that also has 2x 3.5" slots. Trialling it now for a homebuilt Nas, so far impressed. Worst problem is that the bigger drives can be noisy.

Aoostar in the US I think.

Tons. Go look for refurb units from any of the big manufacturers, but I doubt you'll get them at steal prices. Have a look at the Minisforum larger format models that are more updated and $250-400. They can fit that and more. The MS-01 is a gem.