Welcome to MWO!
Even though you say it was answered I'll answer anyway for anyone else's benefit.
Weapons aren't really unlocked. Every player has access to any weapon that the selected mech can equip. This is determined by that mech's hardpoints which there are 3 main weapon types: energy, ballistic, and missile. Each component, or body part, of the mech has different hardpoints. Actual locations do impact gameplay a lot. The other main thing to know is there are two factions, Inner Sphere and Clan. Mechs belong to one or the other factions and carry different faction tech, though there's a lot of similarity in names because both factions came from the same origin (in lore). Mixed tech is not allowed in this game (it was in some other Mechwarrior games)
There are skill you can unlock on a mech per mech basis that enhance the weapons. There's nodes for reducing cooldown, heat, and increasing range. There are no other kinds of skills--there's just a skill tree that is individualistic for every mech you own.
In the mechlab, to swap out weapons, you should see the "warehouse" or inventory list on the right side with various tabs. You can drag and drop weapons and equipment from the mech to the warehouse. It won't allow you to go over tonnage, so you'd have to remove stuff from your mech before adding things back on. There is a handy Strip button on the bottom-right if you want to wipe it clean to only the mandatory components. If you have a Clan mech that is an omnimech, you will also see an omnipod tab in the warehouse that will allow you to swap out components (body parts) and their hardpoints to get a different configuration, but more equipment is locked, like the engine as a tradeoff.
If you don't own something, you will see a price for the mech adjustments you've selected when you go to save it. You have to have that much cbills to make the changes. If you are strapped on cash, you might consider stripping some items from one mech to use on another you own. You can save mechs that are under tonnage, but you won't be able to play them unless they have all the necessary stuff equipped.
Edited by TheCaptainJZ, 29 June 2023 - 10:06 AM.