I wouldn’t bet that the floor is level. Just flat. Cabinets have to be level. And a level substrate for them starts with your framer.

And minimizing gaps also starts with a designer who builds in the versatility of being able to scribe an installation to an out of level floor. Or who orders things in parts and pieces, in conjunction with her installer. Then the toe is custom built on site to physically work with the site conditions.

Opening up a box from China ain’t how a good install happens. Ever.

The back of the cabinet gets shimmed to plumb. If the feet don't touch the floor, they need to be scribed until they do. Cabinet installation 101; beginner level stuff.

Ahh, you're all so tender and naive. I don't trust any of it - that the wall is plumb, the floor is flat, or the cabinet is square.

An out-of-square production cabinet is exceedingly rare. Even if they are out, it isn't usually by much.

The front of the adjacent cabinet in the first photo isn't contacting the floor, either. Looks like there may be shim there. All indications are that the floor is not level and the high point is right next to the wall.

The only way that I can see to fix in this situation and with this cabinet design is remove the cabinets and remove enough material from the back and sides so that the feet will meet the floor without any gaps.

The front gap tells you approximately how much material to remove from the back and sides of the cabinet. I would remove that much and a bit more (1/8") and use these:

https://www.cabinethardware.com/Plastic-Leg-Levelers-p/1329.htm

on the back of the cabinet.

The levelers can then be fine tuned from inside the cabinet. Probably will need to rely on caulk to close any gaps on the visible gable ends of the cabs. Or scribe a template as a pattern for removing the gable end material.