View contents of a second monitor on the primary display
Solution 1:
There are two apps I've seen that claim to give a picture-in-picture type view of the secondary display: Displays and Side Mirror. Neither is free, but such is life.
Alternately, MacOS (since High Sierra, I think) allows you to arrange multiple displays in complex ways. You could add a third display (or an iPad, if your system supports SideCar), and mirror that to your secondary display, leaving your primary display in extended desktop mode with the secondary. Apple's instructions are here.