Hi. I just figured out how to do it with the model you have. Currently you have 2 shapes here that have not been joined by the union feature.
So if you put each shape into a group then hide the group you don’t need to work on, it will allow you to work on the face that you wish to modify. Then you can hide that group and work on the other.
This is the only way I see that you can do it with the current workspace you shared. Below is a screenshot showing how I have 2 groups and which part is in which group.
For instance you could shell the right side and make it hollow with 20mm walls. This will be tougher to do with the left side due to the fillets as it creates the inability for the software to detect the walls/edges.
Let me know if this makes sense.