This frame has specific mounts for weapons; Heavy being for machineguns, autocannons... Main being for, well, basic main weapons like shotguns, swords, rifles etc... and Auxiliaries are either 2 smaller weapons or another Main: 2 pistols, 2 missile launchers, a knife and a nanobot deployer etc etc...
Frames also have certain System Points, systems being sorta special abilities, like deploying active covers, grenades, repair kits, embarked AI systems etc etc...
Frames can have integrated weapons or system, like the Barbarossa having a railgun attached to its shoulder, or a Metalmark having an active invisibility cloaking device.
When unlocking Frames, you also gain weapons and systems of those frames to be used on other mechs. The afformentionned Metalmark can have its invisibility cloak put on a Lancaster, and the Lancaster's winch system can be installed on a Metalmark.
Customisation is mostly akin to Titanfall1 or MechWarriors, the game encourages to do personnal mechs: the mech is basically you in combat, make it as personnal as possible!
