One thing about UEFA Champions League, is that the league is open for every teams to participate in playing the competition, so for any teams to be among the UEFA Champions League, it depends on what each team end with in the previous seasons in their leagues and that is why most teams fight to fall among the big 4 in every league across the countries so that can be allowed to participate in the UEFA Champions League, and this coming season of the UEFA Champions League 2025/26 Season, the teams that are qualified to play the competition knows their selfs and once the league starts, they will surely line up to the game.
You're right, the Champions League is a competition open to any club in Europe that manages to get a ticket to participate in it. But you need to know, not all countries in Europe can directly pass their teams that finish in the top 4 or top positions. Because as far as I know, there are only 6 European countries that can directly pass their teams to get directly into the group stage. These countries are
English league
Spanish league ( Laliga )
Italian league ( serie a)
French league ( ligue 1)
German league (Bundesliga)
Portuguese league
In addition to the teams from these countries, all of them have to fight for tickets to the Champions League through the qualifying rounds. And the Champions League qualifiers are currently underway, so you can see which teams have the possibility to qualify for the group stage and compete in the Champions League next season. So the bottom line is that although the Champions League is a competition open to any team in Europe, it's a struggle to participate in it. That's why the teams that manage to become champions in this competition will definitely be very proud.