Without Git

Compare two copies of a board directly, e.g. two saved versions of the same project. No git repository needed; the older file goes first:

kigit --files owl_v1_old/owl_v1.kicad_pcb owl_v1/owl_v1.kicad_pcb
kigit old.kicad_pcb new.kicad_pcb              # two board files = --files

The schematic next to each board is compared too. The viewer labels the two sides by file name, or by folder name when both files have the same name.

In the GUI

Press Compare folders in the header and pick each copy's folder. kigit finds the board in each (and the schematic next to it), and the viewer opens as usual.

Opening a project that isn't in git offers the same thing, with that project already filled in.