Moving user account manually Had a imac 21" 2011 model high sierra with the nvidia gpu that had high casualty rate, could see from the death it was the gpu it's hd was 2tb.

Bought a mac mini 2012 Catalina 200gb ssd and 500gb hd,
of cause, I can't restore the time machine backup because of the size difference.

My goal is to get my user back with data and keychain and the works, apps do not matter. I took apart the imac and salvaged the 2tb hd and put it in an external usb3 enclosure.

Connected it to the mac mini and it see it and I see all my data on the external hd.

What does it take to restore my user account? Can I make a dir in \Users\ with my user name and mount the external volume as that and then login with the user?

I'm happy enough with keeping the data on the external hd and the os and apps on the ssd.
You could make a different account on the Mac Mini, and if you right-click on the account you created in System Preferences you get an advanced settings pane and could set the home directory to be whatever you want - ie. it could just pick up the one on the USB drive. You'd then be using your old account just like it was before, running from the external enclosure. Be warned - if that enclosed is missing when you log in macOS will create a new one in /Users on your internal drive.

However, in general, all the users settings you care about are in Library in your user's home. So, if you created a new account, and just copied Library over, you'd get the settings you care about. Then could use the docs/data on the remove drive. It wouldn't need your full drive structure - could move those docs to wherever you wanted.