If tools in your Processing Toolbox (or Algorithms in your graphical model) start giving you errors like
ERROR 1: PROJ: proj_create_from_database: Cannot find proj.db
the you can probably fix them by:
/Users/*username*/Library/Application Support/QGIS/QGIS3/
where username is your macOS user name.
Library folder is normally not visible in the Finder.startup.pystartup.py from wherever it landed (usually your Downloads folder) to the QGIS3 folder you opened in step 3.From now on, your QGIS should be immunized against the “fugitive proj.db” problem.
Note: The startup.py contains two lines of python code. It is telling QGIS the location of the proj.db, which should be at /Applications/QGIS.app/Contents/Resources/proj. For some of you the location of proj.db might however be at /Applications/QGIS-LTR.app/Contents/Resources/proj.
You can verify by checking if your /Applications/ folder contains a file called QGIS.app or QGIS-LTR.app. If the latter, then edit the second line of startup.py to reflect this location.