It would be very nice since Esc dont work in this case and when Iam working inside a building is hard to find the background outside to click on it.
Select None is ctrl t
Which shows on the edit dropdown menu.
As several people have already suggested Please make an effort to learn the basics.
Shift/select lets you add or subtract.
how shift would substract?
Using the select tool, hold down the shift key and you see +/-. Pick something then select it again while the shift key is held and it will deselect.
Many of your questions can be answered by spending time at The Learning Center and watching the videos The SketchUp You Tube Channel. The more time spent studying these two sites, will save time by not having to post and wait for answers.
I, too, wanted an easier way to deselect. Go into the shortcuts menu in preferences, choose Edit/Select None and assign a key or combo you like. I’m on a Mac and I chose the back tick (lower case tilde key). I like it because it’s a little easier than Shift/CMND A, it’s right under the escape key (which is what my mind tells me it should be), and it’s not mapped to anything else.