If you look at the video from the point below, it shows the play-off paths being created from D to A - the League B Play-off path is filled by League B teams, as there are enough teams available, with the higher ranking teams participating - the lowest ranked League B team then gets dragged up to play against the three remaining League A teams:
https://youtu.be/slqy1rjQJaY?t=37
And this is confirmed here:
https://en.wikipedia.org/wiki/UEFA_E...ying#Play-offs
So as long as there are enough teams left over in a League, the pathway for that League is filled with teams from within that League, based on rank. It's only if there aren't enough teams left in that League that the best ranked leftover team from the league below is pulled up. And remember, because of the key fact that play-off paths are assembled from D to A, the four best-ranked leftover teams from the lower League will have already been assigned to that League's path, so it would be the fifth ranked leftover team from the lower League that would be pulled up to the upper League's playoff pathway. This way higher ranked leftover teams don't get pulled up to higher League paths, while lower-ranked teams are allowed to play off against teams within their own path.
But a logical extension of this system and principal could lead to the scenario I describe - League A teams are all gone so its pathway must be made up from Lower Leagues; because the League B pathway would have already been assembled from the best-ranked leftover four teams, that would only leave the worst-ranked leftover League B team to be pulled up. Furthermore, as the teams ranked 1-5 from League C would have either qualified already (I've assumed that the League C qualifier is ranked in the top 5) or been assigned to the League C playoff path, then the teams ranked 6-8 would be moved up to the League A play-off with that worst-ranked League B team, to the advantage of that team. this scenario is unlikely, but far from impossible, given that League A contains the teams ranked 1-12 in Europe.
Here's another scenario - what happens if 13 or more teams from League D qualify?