A more specific case is the "sushi belt" where one belt balances multiple ingredients while looping around on itself.
Sometimes they just accept new items in a fixed ratio, sometimes they can actually rebalance if disturbed. This is my favorite https://www.youtube.com/watch?v=7Gt5Zx0bsOQ
While that one uses the in-game circuitry logic, the Factorio forum has a "circuit-free" section https://forums.factorio.com/viewforum.php?f=202
And my favorite fun fact: The "fish" object in Factorio is a useless joke item, but because it isn't used by anything, it's sometimes used as a null value, a flag for when a belt has completed a full loop, or a debugging tool. https://forums.factorio.com/viewtopic.php?p=544302#p544302