Briefs are almost always made out of a polyester material. Squat suits are mainly made of 2 types of material: polyester & canvas. In multiply powerlifting, you can wear both briefs and a suit. You can mix and match materials. Some people prefer poly briefs & a poly suit, some prefer poly briefs & canvas suit. The different materials fit some lifting styles better, but it is largely a personal preference. The big advantage of a suit is the straps which causes it to have more "stopping power" where it stops you in the hole. The briefs provide rebound out of the bottom. It is a subtle difference to explain.
It is feasible that one could cheat and use briefs in raw competition. However, you'd have to put them on and get through your warmups without anybody noticing. If the briefs are tight at all, you won't be able to get depth with lighter weights which should raise some suspicions. I would imagine judges would catch this kind of thing if they were paying attention. And lastly, if you're near record weights, in many competitions the meet director will run an equipment check just to preserve the historical integrity of the records.