Possibly a round gas fire pit insert. Some consuners put stainless steel mesh(no galvanized steel) over the gas burner to reduce the amount of high temp decorative glass needed to fill a space. The decorative glass is expensive especially when getting into the colored/one sided reflective stuff. Looks way better than lava rock.

I'd expect significantly more charring to the inner structure if wood was used and rusting on insert being more uniform in appearance. That round outline plus the orange discoloration on the colored pavers is where many inexpensive steel constructed gas inserts fail. Rain water collects near the valve/orifice that feeds gas into the burner(s), making a low spot in the cover plate. Hard to see this is happening unless covering material is removed to inspect assembly. Usually the damage is found when the fire pit starts up differently.

Its so the quick disconnect fitting doesn't get over tightened into the cylinders valve. Either install it carefully till the black insert starts to flex and stop or break the plastic insert and extract qd with needlenose pliers/vice grips at later point in time.

It doesn't need to be removed from tank to fill it with gas. There's a variety of standardized lp fill valves/fittings that your lp refueling station can push gas through.

Scratch B Gone kit can be used to restore the grain of the finish. Have used it to refinish a marred sst ventilation hood. Its tedious, but gives good results if its bare metal. (A non affiliated link)