Hello, I'm making a hardcore build for minecraft. I want to find out how you can change the chance of items appearing in the chest and change the chance of high-quality items appearing.
That is what I want to do. Various items from mods that are supposed to be rare appear almost at the very start in the chests. In addition, a large number of items are generated in the chests.
I want to change the chance of the items.
How can this be done? I looked at the documentation and didn't seem to find anything that could tell me how to do this.

The recently-updated FAQ covers this in "How do I change the loot".