The board game has extensive rules on mech design, but for weapons, it's just wherever they'll fit. Some of the built-in hardware has fixed locations: cockpit, life support, sensors go in the head; engines go in the center torso (with light and xtralight fusion engines spilling out into the side torsos as well), arm and hand actuators go in the arms, and hip, leg, and foot actuators go in the legs. You can see this in the critical slot section in a record sheet:
https://i.redd.it/jt3p2f1v14gf1.jpegThis one definitely deviates from tabletop: pretty much everything except the smallest weapons have an integral tonnage, and the weapons themselves don't line up directly with their tabletop versions.
The newer games tend to be absolutely consistent with the mech design rules in terms of what weapons are available, and their weights and sizes, such that board game designs are legal in the game (and usually the state you get the mechs in before you customize them), and customized mechs are legal board game designs. They tend to limit the customization by doing things like having mechs have predefined hardpoints that limit what types/sizes of guns can go where, because otherwise you just end up with every mech being identical, with maximum armor and the rest of the tonnage being spent on whatever happens to be the most optimal weapon.
