italian help!! che vs quello che

It's kind of like saying "that" (che) vs "that which" (quello che). Quello che is more specific. Also if you could say "che cosa" then you can't use "quello che" instead.

Che is kind of like an informal version of quello che. Like in English you are much likely to hear "that" over "that which".

It seems to me that you could use quello che in this sentence but it would have a slightly different emphasis.
"la pizza, quello che ho mangiato ieri, era molto buona" sounds more like "the pizza, the one which I ate yesterday, was very good." As oppose to "The pizza I ate yesterday was very good." See how it is more specific but essentially the same.

I don't know if you speak Spanish as well, but if so "quello che" is the same structure as saying "lo que"

I really hope this helps and doesnt make it more confusing