So from what I've googled, from blender 2.8 the reference image thingy is gone, looks like you can add an empty and put an image to use as reference but I keep selecting that object accidentally, what's the right way on 2.9?
The Background Image will be added in the view that you are in, so it is best to be in Front, right, Top, Cameras,...View when you add it:
Then you can adjust Settings as you need them in the Image Tab and disable the selectability in the Outliner:
In case you don't have that in your Outliner, you can enable that here:
Or you can disable selectability for all Empties here: