Mid 2011 iMac as a monitor

A 2020 Mac can't use a 2011 iMac as a display.

Use your iMac as a display with target display mode - Apple Support

"You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display. "

As stated it's not going to work, TDM is gone gone gone with the 2020 iMacs. Buy yourself an external monitor if that is what you want, there is no point in wasting your time trying to get something to work that just won't.

You need a USB-C to HDMI adapter for the MacBook. Just simply connect the two with an HDMI cable.