Hi ErDeIvIt, I'm happy you're liking the game so much! I'll give an answer for vanilla 7 Wonders Duel in this post (without expansions), as I don't know much of the expansion environment.
7 Wonders Duel (with or without expansions) is one of the most played games on the platform, and there are tournaments starting every few days if you look for them. Depending on the torurnament there might be a higher or lower Elo requirement (or maybe even none at all), but normally I think it's not very high. The vast majority of the tournaments are turn based however, so they tend to last several weeks/months, especially the bigger ones. There is no "official" torunaments (i.e. supported by the publisher) as far as I know, so which ones are the most "important"is a very subjective issue.
I know at least a couple of annual tournaments that call themselves World Cup/Championship, and indeed they manage to gather many of the good players. One of them unfortunately began a few days ago:
https://boardgamearena.com/tournament?id=241316
Apart from tournaments, most of the competition is played inside the Arena. There you can choose whether you want to play real-time or turn-based (most people play real-time) and are matched against random opponents to compete in ladder ranking that resets every 3 months.
The last Arena season was a bit disappointing for many of the top players because the account who was awarded the first place did so by cheating (you can find some discussion in a recent thread), but apart from this it is normally a lot of fun and I encourage you to play there if you haven't tried yet.
If you have further questions, I'll try to help.
