Not sure where you're getting 7 postseason teams. I think there's a proposal for it, but wouldn't happen before 2022.
Since 2012, MLB has had 5 teams from each league qualify for postseason play: three division winners and two wild-card teams.
All of these are chosen by a team's win-loss record (with every chance taken to play all games for teams that are in contention).
It is possible for multiple teams to finish the season with identical win-loss records. If separating them is necessary to determine who will advance, then a single game will be scheduled to break the tie immediately after the end of the regular season.
Wikipedia has a list of previous tiebreaker games. Most recently they were played in 2018 when both the NL West and the NL Central had ties for the winner of the division.
If teams are tied, but both would advance and the difference is only to determine home-field, then no tiebreaker game is played. Instead there are several rules that are followed, such as head-to-head record, until one team is granted the advantage.