There are (will be) 12 teams who'll make the play-offs - the 10 runner-ups from the 10 groups, and the 2 best Nation League group winners who failed to qualify via the groups (one is Austria, and the other one will be Czech Republic or Wales).
These 12 teams are divided into 6 seeded and 6 unseeded teams. 3 groups will be created, each consisting of 2 seeded and 2 unseeded teams.
In the group's semis, a seeded team plays at home against an unseeded team. The winners from both group matches will play in the group's final (the home team yet to be drawn). The winner makes it to the World Cup.
All three group winners will go to the World Cup.
As things stand right now, the seeded teams are: Portugal, Scotland, Italy, Russia, Sweden and Poland, with the Poles probably losing this position tomorrow to Wales. The unseeded teams are: Wales, North Macedonia, Turkey, Finland, Austria, Czech Republic.