It's harder to find the brass waste and overflow drains. They had slip joints that could leak later on. Not that you would be apt to notice that in a slab like yours.
1.5" is standard tub drain sizing. The fitting you had for the brass trap would have been a "trap adapter". They carry those in hardware stores. If you use the glue up tub drain, then it's not used.
Having a 2" drain is fine. The plumbers like using a solvent weld p-trap, not one with a swivel. It's just one more location to leak.