Serial numbers and model numbers for fridges and freezers always used to be on the back (as with many other appliances). My own fridge only has its serial number on the back. Fortunately most manufacturers have now had a eureka moment and realised they should be much more accessible.
A model and serial number on a modern freezer should be on a label inside the compartment. It’s sometimes at the bottom but it could be anywhere. Remove drawers from the bottom of the freezer one at a time until you find it.
On a fridge, the serial number is often on the inside wall side next to the salad drawer. Remove the salad drawer to find it. On a fridge-freezer the serial number may be in the freezer compartment.
They are usually silver but can be white, and are usually on the left side.