Warframe

Welcome back, Tenno. So, I'm sure people have made step by step guides, but the reality is that you're going to want to dip your toe into everything you can that interests you. Even some stuff that doesn't interest you.

The most bog standard, but not incorrect, "new player mid term goal" is "Complete the Star Chart". It's something you'll hear people say all the time, but it's for a good reason. They made it such that you HAVE to do main story quests to progress through the planets now, and you'll by nature learn what every new mission type is, start farming some new gear, dip your toe into every one of the bounty zones. By the time you finish it you'll have a good overview of what the game has to offer, as well as unlocking almost all the alternate game modes, including Arbitrations and Steel Path.

I'd recommend building everything you can along the way, every weapon and every warframe, companions, etc. Just getting a lot of that grunt work done so you can cash in on the mastery rank is nice. MR doesn't equate to skill or proficiency at the game, but until MR16 some content will straight up be locked out which isn't a system I agree with but it's the way it works.

To address a few of the things you asked about specifically; Railjack is mostly a content island, you need it for some quests but it's its own thing. Archwing missions are kind of dead old content, but having access to the Itzal and Amesha archwings themselves are nice because they can be used in other content.

Void Keys don't exist anymore, and have been replaced with Void Relics, which is how you get prime parts to sell for platinum or build to get access to weapons and warframes. It's not my favorite system because it reduces all primes to the same one kind of farm, but it does keep it a lively and healthy community so farming those missions is never hard to do.

Tenno Powers aka Focus Schools will be something you largely do passively these days. They're incredibly powerful and useful now post focus 3.0, which while I have my problems with it I think it was overall a healthy change. Certain new enemy types will just give you sizable chunks of Focus when you kill them, so you no longer have to do eidolon hunts to make any real progress. Zenurik used to be the old golden standard because it gave you access to energy regen and that's not bad, but all of them are useful now. Madurai is my standard equip. You can get a good amount of focus just by playing the Zariman missions, but you get a ton from Duviri as well.

PvP is mostly dead, and Rivens are kind of a mess. Rivens can help certain specific weapons, in certain specific contexts, enable builds that wouldn't otherwise be possible, but in terms of rivens for just raw power increase at what the weapon already does they're kind of irrelevant and not something you need to worry about.

I don't know of a good pre-existing newbie guide... so I'll type one up myself:

Top thing is multi-tasking. In warframe it's almost always best to be doing a few things at once since this is a grind-game. So doing more grinds simultaneously or incidentally grinding while doing something else is very good.

As for specific things to do start with filling out the Star Chart and hitting Quests. I'm listing these together because unlocking more of the Star Chart unlocks more quests for you. Do missions until you have a quest, then do the quest (or at least chip away at completing it), then repeat until you're out of Star Chart & Quests.

There's a few things that scale with Mastery Rank (MR), think of it as your account's level, and some weapons are gated behind having some MR level or another. You get MR by leveling up... pretty much everything but only the first time for each piece of gear. So leveling up a spare set of your starting gear doesn't help you here. You'll be given a skills test when you've ranked up enough stuff to reach the next MR, practice the test in a Relay with Cephelon Simaris because some of them are hard and you have to wait a day if you fail outside of practice mode.

Grabbing blueprints off the Market or your Clan Dojo's various Research Labs, building them, and ranking them up is a great way to get most of the MR ranks. This also pairs very well with completing the Star Chart and helps you find what style of weapons you like. After that farming Warframes from the boss fights on each planet is the next best source of MR (and some of the Warframes are legitimately awesome).

Now to address the mechanics you've specifically asked about:

The Railjack is used in The New War (quest) and in the Lich system so I'd recommend putting a little work into yours but you by no means need to master the skill of Railjack use or fully kit it out. Passable skill and OK mods will get you all the further you need.

Archwings are nowadays mostly just used in Railjack missions and for traveling across the Open World areas quickly. Until you get to Steel Path (hard mode) you're really not going to need really any effort put into your Archwing, however Archguns are worth putting effort into because they're used as summonable heavy weapons and the primary weapons for Necramechs.

Void Keys have been retired as a mechanic. You no longer need, or can even acquire, them. You now access the Void through it's connections to Europa, Neptune, and Sedna on the Star Chart. There are however Dragon Keys used to open vaults in Deimos missions... it's worth doing but they keys have nasty debuffs so best to do that with friends/PUGs.

There's a few things made with materials from Fishing and Nightwave will periodically have fishing as one of the weekly tasks so it's worth the minimal effort and moderate time investment. I'd highly recomend grabbing the Oxylus companion from The Business on Fortuna (it helps so much).

Mining... honestly is in the same category as Fishing except you'll want to get mining lazor upgrades from Cephelon Simaris instead of the companion from The Biz.

I thin you're referring to Modular weapons & companions here? Non-modular stuff I've talked about above. Modular stuff is by no means necessary but once your comfortable with non-modular stuff it's worth experimenting with it, making your own Moa/Hound companion, guns tailored to your specific tastes... they're nice.

You mean Operator/Drifter stuff, right? They're wholly unnecessary until you reach the Zariman. They're certainly nice to have and the farming for it is entirely incidental, just slap Lenses on maxed gear that you like and you're off, but you definitely don't need it. Honestly, while the Operator/Drifter is technically needed for some stuff base-level will generally work fine in most situations.

PvP is like 99.99% dead. There's no need to learn it and beyond pure enjoyment of it little reason to do it.

Riven Mods are unnecessary. However they're a great way to give weapons you particularly like a little extra kick. Other than that the Riven market is a nice source of Platinum as unwanted Rivens can be sold to other players for Platinum.

There is a late game that requires you to put in some effort, the Steel Path. You unlock it my entirely completing the Star Chart then talking to Teshin. This gives you access to the Steel Path Star Chart and some nice rewards from working on it. +100 levels on each node, +250% enemy Shields & Armor. It's fun but it can get frustrating with some mission types.

Brozime did a new player experience and has a website that he put a in depth guide of it on at Brozime.com. Id recommend checking it out

IMO you're better off without guides. They usually steer you in a pretty specific direction and often focus quite a lot on 'efficiency', which might not be the most fun experience for you.

The most important thing is setting goals for yourself. It gives you a solid point to work towards, which helps with motivation, and allows you to determine which parts of the game you can ignore for now, which can help you feel less overwhelmed.

A good goal could be unlocking X planet, farming Y Warframe, completing Z quest, etc. As long as it's interesting to you, it's good.

As for learning how stuff works, you should be able to grasp most of the important stuff by just interacting with systems as they become relevant. If you're not sure how something works, you can ask in the in-game Q&A Chat.