I’m looking for a symbol for a rectangle or triangle that would indicate an angle of 90 degrees. I believe I saw one for 45 deg.
The character symbol used to indicate a right angle is ⦜, unicode character 299C (hex).
To indicate right angles in a geometric shape, draw a small square where you would use an angle arc.
LibreOffice has it with its own keyboard shortcut for this purpose (which means you should be able to do it on any OS)
So the recommended way is typing 299c then Alt+x and you got the ⦜ symbol
You can also select in in the top menu Insert then Special Character... and choose the one you want, or directly from the tool bar click the Ω sign to open the special characters window (Click on More Characters)
Another way to do it is to use Ctrl+Alt+Shift+u then type 299c then Enter immediately (this option is less portable, but should work on Linux (at least my distro) and Windows)