How to isolate a refrigerator?

The floor is not the only part the vibrations may be entering. The walls next to and behind the frig may also be adding to the noise. If you dampen them also, it may add to the quieting.
Along with vibration pads under the frig feet should do the trick

my <1year old fridge is LOTS quieter than the 20 year old GE it replaced...and has cut my electric bill by about 50kwh per month while being about 10% more capacity.

IF your current fridge is more than about 10 years old, and IF the sound deadening ideas don't cut it, you may consider a new box.

On my brothers boat, he sometimes will put a small (2kw) generator on the upper deck. Adding a 2thick of those large foam floor tiles made it work and cut genset noise drastically. Tiles available at any 'home despot' type store.

FWIW... A gas refrigerator has no compressor and makes no noise. They work fine and are usually sold to folks who are "off the grid".

I checked out the Liebherr 'fridges when we were looking for a new one. They are very nice German products, but a bit unconventional in that they are very tall and don't fit in the average American space allocation for refrigerators. They have no fan motors, so they are quiet. Performance comes at a price though, starting a $5,000 for the least expensive unit at the retailer I consulted.

We had a gas fridge on our boat a long time ago. It ran on bottled alcohol and needed a flame. I house unit could probably be plugged in for the heat source.
Our boat unit? give it 24 hours untouched and it'd freeze stuff solid. I didn't appear to have a 'speed' control

No fan is nice, but no compressor would be better.

newer fridges are much quieter than old.