For the modules:
This is due to the rareness of the item. Legendary torso are great, if you manage to get a legendary torso at any level, I would advise you to build your mech around that level. They have great base stats and they still get as many (in cases more) module slots than other torsos. Even then, there are some epic and rare item torsos that can still compete well. For these you just have to push everything to edge as much as possible (EG, If you find yourself not needing to cooldown at all in a match, then you have too much heat on your mech and you could switch it out for a armour plating module.)
For the resistance, the most common is to use the multi-resistance kit as it only takes up one module slot. However in MrOneTwos case, as he's had lots of experience with the game, he's found a battle style where he can afford to use 3 separate resistance modules and still manage his heat and energy. It all comes down to experience with your mech really and pushing it to the edge.
As for bullets and rockets, if you look at two of the builds you have shown they don't use them. This is a smart move, as weapons that don't use rockets or bullets are just as strong as ones that require them (most the time). So why bother docking one of your module slots for bullets and rockets, when instead you can use a weapon just as strong that doesn't require a module slot to be taken up.
Physical weapons do A LOT of damage, even at lower levels. If you want a strong low level mech, you should definantly have a few strong physical weapons on your mech. Again it helps if you have legendary weapons, however this isn't as much of a problem as not having a legendary torso (in my personal opinion).
If you look at MrOneTwo, he has all physical damage weapons. 3 of his side weapons are epic and they do between 45-55 damage without fusion. With fusion he's able to do a lot more (and with those dark reds ones adding, I think, +9 damage he's able to achieve 65 damage).
His top weapons are the most advisable tops weapons for low level mechs. They do 40-50 damage and best of all, they don't require tokens to buy them, so you're able to purchase them in the shop.
MrOneTwo's build is all about doing as much damage as possible as quickly as possible.
Brasil is the same. Again he has all the high damage weapons that MrOneTwo has (excluding the sword) so he can get a lot of damage done quickly. Only difference is he has a big energy draining weapon. If he depletes your energy, you're unable to use any swords or energy weapons yourself that turn. Most importantly though is you miss out a turn on your drone (possibly more as there aren't many energy builds at lower levels, so most people neglect energy modules).
In all he does high damage, on top of reducing the damage received to him.
Pylindafa99's build, I wouldn't recommend. If it works for him great, but to me it doesn't look like a strong build. RPG's are fairly useless, they take a lot of rockets to use and they aren't amazingly strong I don't think. The only plus about using an RPG is you're able to deploy your done and push the enemy out of range in one turn, which isn't really worth having the weapon.
This mech however does have a lot of heat weapons. So I imagine this guy overheats most of his enemies and uses his bullet weapon to get in some stronger damage. Best thing to do when facing a heat mech is remember, heat weapons are not strong. Don't be scared to cooldown against them because the damage they do won't be very much compared to a damage build. Cooldown, hit him with a weapon and let your drone hit him. If you need to double cooldown then fine, at least your drone still hits him. Just never let someone like this force you in to a double cooldown, because then you'll start having problems.
It's a long essay I know
I hope it helps though