Video Editing app for 2012 iMac

That machine is obsolete. it is no longer supported. It is not capable of running any supported Operating System.

If you don't already have a video editing app for it, you're not likely to find one at all any more other than maybe an old build of an open source editor.

FWIW I used the old iMovie versions 1-6 to edit DV/D8 movies on System 9-Leopard-Snow Leopard-El Capitan-Mojave on PowerPC-Intel Macs. You might get iMovie HD 6 from eBay (iLife 06 installer doesn't run on OS X 10.7 or later because it is a PowerPC application, but iMovie HD 6.0 can be installed via Pacifist and updated to 6.0.3).