#ModdingTipOfTheDay: To keep TE info in the NBT of your dropped block (to remember stored items) you have to do tricks because by the time getDrops() is called the TE is already gone. Use this magic to avoid this and gain the ability to get info out of the TE in the drop(s):

Is that how Shulker Boxes do it?

Unfortunatelly not. They use spawnAsEntity() in breakBlock() which means that this is not in getDrops() making it harder for things like quarries to gather these drops