That assumption is wrong.
Next June the final will be held between the winners of the groups. So that’s before the qualification for the euro has ended.
In march 2020 they will see which group winners did qualify for the euro. The ones that didn’t will go to the play off, if the winner did qualify it will be replaced by the runner up in the group.
I don’t know what is going to happen when too much countries within a division did qualify!