It’s kind of dispersed, for example, you can check this thread also: Big List of Defold Pro Tips! and generally The Defoldmine category on forum
and of course: https://defold.com/assets/
It depends on what you’re looking for, many generic things you will be able to find in the internet (many times you don’t need to search for Defold specific solution only, there is a lot of Lua libraries/modules) if not, just tell us on the forum what you need 