You're looking for special chests with glowing lights, which make a sound when you get close to them (sort of like the Nirn root sound from Skyrim, if you're familiar with that).
They are an absolute pain to find if you don't have any Loot Radar mods equipped (Animal Instinct, Loot Detector, Thief's Wit), so ideally get one of those if you're doing bounties on the Plains.
There are two areas however, where the chests aren't in that zone, but in caves under that zone, so look for cave entrances nearby if you can't seem to find any caches.
Level of your equipment (weapons, warframes, sentinels, etc) only dictates the capacity (mod space). Your actual 'level' is your Mastery Rank, which currently goes from 0 - 27.
Even then, the power of your weapons is more reliant on how levelled up your mods are - without decent mods, a level 30 weapon is not more powerful than it was at level 1.
The more powerful weapons and warframes are locked by Mastery Rank, but everything is endgame viable with enough Forma and reactors/catalysts, so if you really like your Mk-1 Paris, you can take that on end game content (I have default colour Excalibur with disguised starter weapons to prank people).
Defence missions are quite good for farming mods and loot at low levels (just not the Earth one as the pathfinding is knackered on that).