Why does the freezer suddenly have more frost than it did

Yes.

If the device has separate compressors for the fridge and freezer then it's perfectly possible for one to fail and the other keep working.

Generally the cheaper the fridge/freezer the more likely it is to only have a single compressor.

Usually there's a fan that moves air from the freezer to the fridge. It could be that the frost is preventing the fan from spinning and/or blocking the air chase between the two. Would probably take a week or so to build up to that point. If the defrosters not working, which, if that's the back of the interior of your freezer, it looks like it isn't, that's definitely what I would start checking.

I'm not fridge guy, but my first step would be to verify voltage to the defroster.