Patched inner tube helped by pressure against tire? When I patch a tube, does it need to be in a tire to "remain fixed"?

I mean: I understand an empty fixed tube will remain fixed if I keep it folded in a box...

But how about with air inside? does the patch need the pressure against the tire to remain steady?

I have pretty much never patched a tube,... so not really confident about the repair yet...
Is it a good idea to ensure the quality of the repair by inflating the fixed tube and leaving it out there for a few days and hope it keeps its pressure?
It's not necessary for the tube to have the external pressure of being installed in a tire. If patched correctly the patch will hold just fine if there are days or months before it is used.

My normal habit is to travel with a spare tube. When I flat, I switch the tube out, and carry the old one home where I patch it later. In my garage I have a box of patched tubes that I reuse as spares and the patches always hold.

If you wait a few minutes after patching, you can, as you ask, fill it with air outside of a tire, and the patch should still hold. In fact doing that and then dunking the tube in a basin of water can be a good way to test that the patch was installed correctly before you deflate it and store it for later use.