I can't really speak to that CompoundTag. I didn't know it existed until this question. 🙂
From what I can see it is used to pass additional ItemStack NBT for the SpawnEggItem and ArmorStands?
It will be up to the particular Entity implementation how it uses it in its finalizeSpawn() method. Also see the spawn() method that takes an ItemStack.
Nothing in vanilla uses the spawn() method you are using directly.
But, it looks like that same method you are using has a Consumer<T> that let's you modify the Entity before it is spawned.
e.g. something like:
ModEntityTypes.NEXUS_ZOMBIE.get().spawn(level, null, zombie -> zombie.setNexusTarget(getBlockPos()), spawnPosition, MobSpawnType.EVENT, false, false);