I have a couple questions about schematics.

How would one make random loot in schematics when spawning a custom structure.

How could I make my custom mob spawn only in the structure without spawners. like the witch / witch hut.

I don't have access to a computer atm so I can't look at its code I am using my phone. thanks in advanced.

Define what is your "schematic" (what data structure) and what you mean by "spawning structure" (on generation or per command/action or what).

 

Putting random loot into chest is a matter of simply putting it into chest's TileEntity when chest is detected during structure-spawning. Question is when do you want structure to spawn. There are also events that might help, but you need to be more specific.

Re random chest contents:

Go look at

ChestGenHooks

You just code it. If you want to spawn an entity in the structure, then just spawn one. If you want items or tile entities with loot, just place them.

 

Basically, in your schematic you should have indications on where possible entity spawn points, or loot points are, and then you would use standard Java to check random chance and pick loot or entities or whatever to place.

If you are talking about a .schematic file such as one produced by World Edit, you need some code to handle loot population in the chest and manage the mobs.  I built a mod that does just that - it uses schematics to generate village/world structures, ability to generate loot inside chests of that structure, and to properly handle entities that may be saved with it.  The code is on github if you want to take a look at how I did it.