At least in Europe, there is a very annoying beep if you don't wear the seatbelt. You can still drive, and there are ways to disable it, but most people just wear their seatbelt.
I don't think it is mandatory, but it counts in the EuroNCAP score, and since it is one of the easiest safety feature to implement, they all have it.
The beeps in the US are not that annoying. Ding ding ding ding ding for 30?seconds at start up, again when you put it into drive, and then again when you hit a certain speed (around 7-10 mph) and/or periodically. It's not pleasant, but it's tolerable when I'm moving cars around between my house and my barn. Yeah, I probably should still buckle up, but it's not critical for sub 15 mph, private road driving for a minute or two.
My first car didn't have a seatbelt reminder, and it needed a bit of time to warm up, so I got in the habit of starting it and then buckling, and 20 years of driving with seatbelt reminders hasn't trained me to switch the order.
You are right there are ways to disable it easiest being a seatbelt delete that clips into the buckle and disabled the annoying beep and you can clip into that should you choose. Who would choose to not wear a seatbelt is strange to me but I also rode a motorcycle so we are dead meat anyways if something goes wrong.
US cars are the same way. If you move faster than 5 mph (which is really slow) and aren't wearing the seat belt, the car will start with uncomfortable beeping. It eventually climaxes to a nigh unbearable screech.
I don't think it is mandatory, but it counts in the EuroNCAP score, and since it is one of the easiest safety feature to implement, they all have it.