Freezing halts spoilage but does not leave food unchanged. The damage is mechanical, it happens during the freezing itself, and its extent depends on how quickly the food passes through a narrow temperature range.
Ice crystals grow and puncture cell walls
Water inside food forms crystals as it freezes, and those crystals expand as they grow, pushing against the structures containing them.
Cell walls and membranes are punctured by that expansion, so their contents leak out when the food thaws rather than staying where they were.
This is why thawed produce weeps liquid and goes limp, and why the loss is permanent no matter how carefully the thawing is done.
Freezing speed determines crystal size
Slow freezing gives a few crystals time to grow large, and large crystals cause proportionally more structural damage.
Fast freezing produces many small crystals instead, which puncture far less and leave a texture much closer to the original.
Commercial equipment freezes far faster than a domestic freezer, which is the main reason bought frozen vegetables often hold up better than home-frozen ones.
Water content predicts how well something survives
Foods that are mostly water in delicate cell structures, such as salad leaves and soft fruit, are damaged severely and cannot be restored.
Dense, starchy and fatty foods hold up much better, since starch and fat networks are not dependent on intact cells for their texture.
Cooked dishes generally freeze well because the cell damage that freezing would cause has largely been done already by cooking.
Temperature fluctuation is worse than the initial freeze
Every partial thaw allows small crystals to melt, and the water then refreezes onto larger crystals, so damage accumulates over weeks in storage.
This is what causes freezer burn, where surface moisture migrates away and leaves dried, discoloured patches with a stale flavour.
Keeping the freezer closed, filling empty space and wrapping tightly all reduce the fluctuation, and they matter more than the total time stored.
Some changes have nothing to do with ice
Enzymes continue working slowly at freezer temperatures, which is why vegetables are blanched before freezing to deactivate them first.
Fats oxidise over months and develop stale flavours, which is why fatty foods have shorter practical freezer lives than lean ones despite being safe for longer.
Freezing preserves safety far longer than it preserves quality, and planning around the shorter of the two is what keeps frozen meals worth eating.