I want to get a bicycle that can sustain more than 120KGs. I know most of the bikes can sustain a maximum of 120KGs, but how can I maximize that limit? What components should I change in order to make the bike more rigid and not prior to breaking underneath me?
Are you planning to buy a mountain bike if you're into mountain biking (i.e. riding at very high speeds on bad terrain) or because you think that due to your weight, a mountain bike would be the best kind of bike?
If you're planning to buy a bike to get from place A to place B or just to enjoy riding on roads, you should really consider a loaded touring bike and not a mountain bike. Stay away from so-called "road" bikes because they are only intended for featherweight riders. They should instead be called racing bikes.
The thing about loaded touring bikes is that if you buy one that can withstand 12kg + 12kg dual panniers on the rear and 8kg + 8kg dual panniers on the front for a 100kg rider, it means equivalently that it can withstand a 140kg rider. That's plenty for your weight. With your weight, it also allows 20kg load, i.e. you can use it for grocery shopping too.
A mountain bike isn't the ideal kind of bike on roads. If you really start to do mountain biking, I expect that 99% if not 99.9% of mountain bikes sold on the market that are intended for lightweight riders are insufficient for genuine mountain biking for 120kg rider weight. However, if you ride very carefully then it may withstand 120kg weight, but having to ride very carefully may remove the enjoyment from mountain biking.
Any wheel for your weight should have no less than 36 spokes. If the wheel is poorly built (unequal and/or low tension and/or not stress relieved), you may need to have it retensioned by a competent wheelbuilder. There's no reason why a competent wheelbuilder couldn't tension a 36-spoke wheel to be durable for 120kg rider weight. Any less than 36 spokes and the likelihood of the wheel surviving even with a perfect build are lower.
To maximize the limit, buy a bicycle with the biggest limit found in the shop. Do not buy a bicycle with no limit specified. Do not try to guess from specific features as it is difficult to name the weakest part. If the wheels can support lots of load, maybe the frame cannot. If the frame would be OK, maybe the brakes are not adequate.
Why it should break, if you and your cargo together weight 120 kg and the bicycle is rated for 120 kg? You are using it appropriately.