It’s basically a movement with various different groups with similar goals, but no central organization. For example, there is the /r/buyfromEU subreddit, whose mod team has also created a Mastodon account, but who don’t run any of the Lemmy communities AFAIK. A couple of users from the subreddit created the GoEuropean app. Various users also created infographics, which get shared around between the platforms. Lemmy has 2 major communities, this one and !buyeuropean@feddit.uk. There’s some smaller offshoots from users who either didn’t like the instances these are hosted on or had some other issues with them I guess. It’s indeed all a bit fractured right now. As for where to post, pick any that’s active I guess. I’m subscribed to both this one and !buyeuropean@feddit.uk, but I slightly prefer the latter, because I think “Buy European” sounds better as a slogan and explicitely includes Non-EU countries like Switzerland and the UK, which I’m in favor of.