I have tried to use https://mtgarena.pro/ or https://mtga.untapped.gg/ or https://mtgatracker.com/. These seems to be only tracking my games wins & loses status. They don't seem to know what my card collections are. Source: over 1 year ago

Others would know better than me. Here are a couple I've seen, but haven't used. Not even sure if these are free or not: Https://mtgarena.pro/ Https://mtgatracker.com/. Source: almost 3 years ago

I’ve been using https://mtgatracker.com but it’s been having trouble / not telling me everything that’s actually happening. Source: about 3 years ago

There are a lot of deck resources for Magic. MTGGoldfish, MTG Melee, MTG Top 8, MTGAZone, AetherHub, etc. These won't all be Arena-focused, some specialize in tournament decks, others have user-submitted decks that may or may not be worth building, but the major thing to look out for is budget decks for tabletop won't necessarily be budget on Arena as in paper certain rares and mythics may be dirt cheap while on... Source: over 1 year ago

Arena has better third party tournaments when it comes to prizes. You can find these tournaments on mtgmelee.com. MTGO has had great third party organization too, but typically they are free to play tournaments with small rewards, while mtgmelee tournies have entry fees + prizes or in some cases free with prizes for big promotional events. Source: over 1 year ago

Check out this video, https://www.youtube.com/watch?v=yW-ZL2rQAZI In addition you can join free tournaments on sites like https://mtgmelee.com/ that have cash prizes for top finishers. Source: over 1 year ago

Deck submissions are required through mtgmelee.com for the Legacy Main event and Modern Memorial. Deck entry will be available a week or two before the start of the event date. Source: about 2 years ago

Tournament decks mostly, which then get picked up, tuned and promoted by streamers. mtgtop8.com, mtgdecks.net and mtgmelee.com are good starting points. Source: over 2 years ago