The gist there is that there are two leveling systems in Warframe: Your stuff's individual level, and your account level. The first is self-explanatory, but the second, your overall Mastery Rank, needs a bit more explaning.
Your Mastery Rank governs two things. Firstly, it acts as a requirement, for example IIRC highest MR requirement is for the Syndicate Melee weapons at requiring MR 12. Secondly, nowadays MR also determines your minimum Mod Capacity, meaning that even unleveled weapons or frames have at minimum the Mod Capacity they would have if they were the level of your Mastery Rank. Note though that this does not grant you any new Mod Capacity, it simply makes leveling stuff easier because they come "pre-leveled" (ie, at MR 20 your stuff starts gaining "new" mod capacity only at level 21). MR also determines the maximum number of Extractors you can deploy at a time, your Standing per day maximum, number of free Loadout slots you have, Void Trace maximum capacity, and number of trades you can do per day.
Unlike usual leveling, your account's Mastery Rank is not increased by repeatable tasks or things like killing shit. MR is increased by one-time accomplishments, for example clearing a map node for the first time, unlocking Junctions, but most importantly of all every new level you get for each specific type of equipment for the first time. The amount gained depends on whether the equipment piece in question is a Warframe, Archwing, Companion, or Weapon. Warframes, Archwings, and Companions each give 200 Mastery Rank xp per level, meaning the first time you level a Loki to 30 you will gain 6000 Mastery Rank XP. Skana and Braton MK1 would be worth 3000 each at level 30. But remember, this is ONLY first time each level is reached with that particular type of equipment, meaning even if you built an all new Loki you would not gain any new Mastery Rank xp from leveling it. The same applies to slapping Forma on anything, you won't gain any MR xp from releveling equipment, though this is something you will end up doing with the good shit you amass since Forma are the next step up in the power curve (for example any frame I would call something I'd actually use generally has around 6 forma, depends on starting polarities type and number). But it's also important to note that Loki and Loki Prime are DIFFERENT pieces of equipment for MR, as are Braton, Braton MK1, Braton Prime, and Braton Vandal. Every variant of any equipment counts as new MR xp.
This is where scrapping junk equipment comes into play. You will have minimal storage space for your shit (even if they are clever at DE and sell inventory slots as some of the cheapest stuff in the cash-shop to rope you in to spending money), so you don't want to keep useless crap lying around after its real purpose (MR xp) is fulfilled. So something like Braton MK1 should be tossed right out the airlock after it hits level 30. Skana though should be used to build Dual Skana, which after hitting level 30 should be used to build Redeemer (you want to be aware of if a particular crap weapon is used as an ingredient for crafting another weapon to save time and effort).
Nonetheless, there are a fair deal of good to great weapons et al in the game, so you will start encountering problems in this regard, since a lot of stuff you don't want to or objectively should never ever airlock. Frost Prime for example is so valuable that you don't want to scrap it ever. Most older frames overall have a "Prime obsolescence" to them nowadays, so once you get your hands on Loki Prime you can airlock the regular Loki, even if Loki->Loki Prime is the most meager improvement from regular to Prime for just about anything in the game. That's really where the hard part comes, since rare and good often go hand in hand, and you have overall limited space (discounting buying new slots, which is initially a very good investment if one wants to spend cash or do some Trading for plat), so the weapons et al that are NOT shit will start piling up dangerously fast once your account starts hitting critical mass of cash and resource flow and ability to rapidly level stuff.
