It is a grindy game, eventually you will need to make use of platinum ( premium currency) to unlock slots for warframes, weapons, pets or to unlock skins and convenience items like forma - which can be farmed but fully completed ones are a rare drop and to build 1 takes 23 hours- ( item used to allow great customisation of frames and weapons and resets forma'd object back to level 1) and there are a few other things spending platinum on mostly for slots, but that is in spoiler terriority to mention.

One can farm stuff most of the stuff if you have the time and patience, besides the "prime" version of mods and weapons and warframes.

Prime stuff is essentially the "best" version of something and considered "best in slot" for the space it takes( prime mods are a bit more complicated in regards "best")

The prime mods one gets from a trader that pops up every 2 weeks and exchanges currency that is earned by cashing in prime parts for currency and use the currency to buy from the trader.

Prime warframes and weapons are rotated over time and put in a "vault" when its time has run its course. There are a few primes that not vaulted, but for the most part one just has to wait if they are looking for a specific prime.

Platinum can be traded for items, but is probably best to trade platinum on the unofficial market site.

Generally it is a good game with good gameplay, but is very, for lack of better word "gated" by a lot of timers and daily caps that either the impatient can use premium currency push through either by buying the stuff they want or speeding up the forging process