I dont keep a pond and my question isnt really about ornimental ponds but i was thinking about this. How come that fishing ponds that dont have filter and are full of messy fish like carp, how come the ammonia dosnt build up and kill allt he fish?

It is how the ecosystem is. All the plants, bacteria, etc remove the waste in the pond and it is a much larger area so it will much harder to pollute.

When it rains it will also probobly help to add cleaner water