Custom loot Tables
Loot tables are powerful tools that define what block drop upon destruction, what entities upon death, what equipment can entities use, what can a player fish out of a river and what loot ends up in the game's loot chests. In order to understand loot tables better, I recommend looking into the loot_tables folder of the Example Vanilla Behavior pack, but I'll trow in some examples from there into here nevertheless (you can scroll down or to the sides for better readability).
One can even make a loot that drops half-removed blocks like glowingobsidian and netherreactors!