found nothing too, but on the othere hand even the archarbor puzzle is disabled, so there might be something, just disabled too

It looks like the whole conundrum of the issue is that Reddit own mobile website or mobile apps are not correctly showing the message on the closed sub, but only that it is now a private sub, see https://dormi.zone/comment/90345

So I see the issue is what the mobile website/app shows you

This is what is visible at https://old.reddit.com/r/Warframe/ on desktop browsers:

And this is what is visible at https://new.reddit.com/r/Warframe/ on desktop browsers:

Then you should complain to Reddit for hiding you useful information to you, simply because doing that it is not useful for them, and not complain here to dormi.zone

Also wait the 30th of June when "where most users are", that is on mobiles apps and not on the mobile website because that one sucks badly, won't be able to see reddit at all, not only one sub

Not that I'm aware, but then I also don't like too much watching video guides unless they are on a very very specific topic and they are also very very concise.

I play since closed beta but I'm on and off ...even for loooong periods of time, like as 3 years in the case of my last hiatus, and for anything I don't know I just search the wiki: https://warframe.fandom.com

As for your questions:

Railjack

Don't mind about it for now, you can play also without by joining other crews. It used to need belonging to a clan to have one, don't know if it is still the same (my clan luckily never kicked me for inactivity so I don't know)

Archwing

Most of it got integrated into RJ, while AW Guns can be used temporarily also in open world missions, or mounted on .... another thing

Void Keys

They exists no more and they have been converted in relics. Each relics has a specific loot pool disregarding the mission they are played with. In practice DE moved the prime loot pool from specific mission types and planets to specific atrociously named relics

Fishing

It is an activity you can do in open world maps, it can help you level up the relative faction and by doing that you usually can also improve its possibilities too; mats are used for crafting gear

Mining

same as fishing

Building my own weapons/pets/robot companions

They rely on levelling up the open world factions, needs fishing/mining ingredients and also conservation ingredients too sometimes. That is another activity like hunting but non-lethal and finalized to the safe capture of endangered species.

Levelling up my Tenno powers

Focus? Faster farm afaik now is doing zariman missions, but you need to finish the quests in order to open that content

Conclave PVP

what?

Riven mods

I never use them or feel the need for them, I don'0t think they are needed in any content, but it might be good way to get some plat by trading them

and I'll add Duviri if you picked that way to start:

it is a kind of rogue-lite game mode, quite simple and handy for new players.

As for in general I'd focus on unlocking the star chart (and the relative junctions!) and playing the quests. Have fun tenno.

I've missed last rotation so what I should go for 1st? I'm thinking Braton and Skana for the prime and vandal but I'm not reading great about Skana.
Then maybe Paris, but I've got Dread already and they look similar in their incarnon form or not? Also have got Despair too so kunai seems redundant and weak with no prime variant...
So prolly I'll go Lato...