What on earth has one to do to join the Belgian team?
There was a sign-up form posted to this forum and the Dominion Discord in November. You filled it out, including what country you wanted to play for. The top 4 players for each country, as decided by your rating in Shuffle IT, were formed into a team. Countries that didn't get 4 sign-ups were merged when possible - for example, there's a West Europe team.