There are a few factors that decide what type of loot you can find:
- World tiers 1 and 2 will never drop ancestral, sacred or unique items https://diablo4.wiki.fextralife.com/World+Tiers with the exception of for example https://diablo4.wiki.fextralife.com/The+Butcher's+Cleaver and https://diablo4.wiki.fextralife.com/Mother's+Embrace
- The rarest group of unique items can only drop from level 85+ enemies https://twitter.com/AZJackson85/status/1672465211159363584?s=20
- Each monster family is said to have a bigger chance of dropping certain items. This can in theory be used to your advantage to find a specific type of legendary item https://maxroll.gg/d4/resources/monster-families
Other than that there are no known limitations or benefits of loot tables between enemies, or logic that gives you better chances at better gear, such as kill speed. All we can assume for now is that the drops are random, with the above rules as limitations.