Can't adjust UI resolution in PS 2021. on 2020 iMac with Retina display

When you say “everything is very small…even at 100%” is that just the image content, not the tools and menus? Or does that mean everything in all applications?

If you just mean an image in a Photoshop document window, that’s normal and expected. Because 100% is defined as one image pixel to one display pixel, and a Retina display has much smaller pixels, that naturally means 100% looks smaller on Retina displays. This is technically correct, so much so that you will find this to be true on any Mac image editing application on a Retina display (and on any Windows image editing application on a HiDPI display), not just Photoshop.

If you wish 100% meant “real world size” instead, that’s a different command; if you are designing based on inches, choose View > Actual Size instead.

If you are designing for screens, then yes the workaround is to view at 200%. Because web browsers automatically scale images by 2x to compensate for the smaller display pixels. In other words, if you take a screen shot of a 300 x 200 px image in a web browser on a Retina display and you open it up in any image editor, it will measure 600 x 400 px.