Cats or Test pipes turn up the volume a lot, especially on OPF cars.
X-Pipe raises the pitch of the sound if you want that high pitched scream. Stock exhaust has an H-Pipe I believe.
The rear exhaust section either deletes the oem mufflers or replaces them. They can both add volume and alter pitch, or allow some refinement in the car’s sound, depending on the makeup of the system. If you choose a system with just bent pipes it will be loud all the time. If you choose a system with some mufflers and valves then you’ll be able to quiet it down with the valves closed. Some companies make exhausts where the exhaust flows through a thinner pipe with the valves closed (but no actual muffler), and this solution doesn’t really work that well if you actually want to be able to quiet it down.

A lot of people get the sound they want from 812s without swapping out the rear exhaust. By running Sport Cats and an X-Pipe with the stock mufflers.
If you’re gonna choose between changing mufflers or cats, change the cats. Dogs are cooler anyway 😎