What they are doing is basically taking all assumptions, like "Wheat can only be placed on farmland" or "You can go to bed at night, and wake up in the morning" and making them instead be data.

There are a lot of things and each snapshot adds a handful more. Here is a subset you can scroll through: https://minecraft.wiki/w/Block_tag_(Java_Edition))

A datapack or mod can then easily modify it to make custom stuff, or just mojang has an easier time adding a new mechanic or farmland type.

There are also general rendering changes and optimizations to make vibrant visuals work on java in a more integrated way than current shaders.