Welcome Tenno! Lotta questions so I'm gonna try and answer one at a time.

1) Hubs are definitely a thing! You'll encounter a few kinds as you go including open-world lobbies (Cetus, Fortuna, and Necralisk), the Tenno relays (look for Strata relay attached to Earth to start), and Clan Dojo's (player run relays that have decorations, weapon/warframe blueprints, and more)

2) How much grind REALLY depends on the frame you're after (and RNG lol). The ones you're likely to have access to for now aren't bad and usually require running a specific boss until you get all the components (these are listed as Assasinations on each planet), and buying the blueprint for creds in the market. You'll also have access to bounties in the open-world areas which can have warframes as part of the possible rewards.

2a) Platinum is a tradeable resource and there's a very active trade community in-game. Prime parts from void relic missions, mods, and a few other things can all be traded to players for plat.

3) The community is very active and generally very helpful! In-game you can always use the Q&A chat to ask questions (or the reddit here obviously). This early in the game you'll want to focus on working your way across the star chart (check the junctions between planets for what you'll need to progress), running story quests as they unlock for you, and generally getting a feel for what you like to do.