Fiberglass is FRP. There's absolutely 100% no difference between the two. FRP is just a general term used.
Fiberglass is glass....it cracks a lot easier than polyurethane.......
you cant even push it.
If fiberglass takes on so much as a pebble,
it will crack and shatter where that pebble made contact.
There is no fixing that other than to glass over it.

Polyurethane is much more durable and can be sanded/repainted
if over time there are rock chips and the likes.

Do a Google search and see for yourself:
frp vs. polyurethane

OEM bumpers are fiberglass or plastic?

Plastic.
Most aftermarket body kits with a few exceptions are made of fiberglass.

OEM bumpers are fiberglass or plastic?

OEM bumpers are fiberglass or plastic?

Polypropylene. In other words, flexible plastic.

The OEM Lightweight wing is not made of fiberglass.