HyggeState Yeah so, as someone who has been playing for 20 years, I am also confused about the formats right now. I will try my best.
The main ones in general paper magic are:
Vintage - Everything
Legacy - Everything within reason
Modern - Everything since 2002
Pioneer - Everything since 2012
Standard - Everything since 3 falls ago (as of this very set they changed that from 2 so even this one is in flux despite the name)
There are also casual formats like Pauper (only use commons) and Commander (100 unique cards and a legendary creature you get to cast at will); and limited formats like draft and sealed which give people a random assortment of cards from a single set and have them build a deck from those.
The problem on Arena is that the really old cards still aren’t in the database yet. There’s 30 years of one off mechanics and 50,000 cards to code so it’s going to take years to reach parity with paper. So while Arena has the more recent formats like Standard and Limited, it has to make do with an “all cards on arena” format called Historic….or that is until they started introducing digital only cards that could do things you couldn’t do in paper (for example cards that would draw a random creature from your deck) that are sequestered into their own specific formats. These cards are called Alchemy cards and there’s a version of standard called Alchemy that allows these cards as well as balance changed versions of problem cards in Standard. All well and good but these weird cards are on Arena and therefore they are in Historic, and nobody liked that. So they made yet another Arena only format called Explorer, which is everything on Arena that also exists in the Pioneer format in paper. This format is rapidly approaching parity with Pioneer, but isn’t quite there yet. Once it is it will presumably be removed.
Oh and also there’s Brawl, which is like Commander, but on Arena, strictly 1 v 1 rather than multiplayer, and there’s a version of it for every Arena format.
(Takes breath) Ok where were we? Oh! Right, the lord of the rings cards. That set is legal in Alchemy and Historic only on Arena, and is legal in Modern, Legacy, and Vintage in paper.