I gotta be honest here, every time this left/right wing argument comes up I question if people actually watch Dortmund play or just look at the position on the spread sheet
For over a decade now and under multiple coaches (be it Klopp, Tuchel, Favre or Terzic) one trademark for Dortmunds attack was always fluid positions in the final third. The offensive midfielders, especially the wide players are not supposed to hold their flanks the whole time (that is largely the job of the full/wingbacks) but are supposed to roam and interchange frequently. Nowhere can this be seen as clearly as when Sancho and Reus share the pitch. They cross each other frequently, creating overlap on a side to shift attention of the opposing defenders and open spaces that way. They also don´t have a side they favour here to lessen predictability which leads to a very balanced offensive approach. Over the last season they build up the exact same percentages of attacking plays over both sides (38%), which is actually rather uncommon as teams usually prefer one side over the other (United for example builds up more over the left side). Sancho plays the side where he can have the most impact depending on the situation.
As for this whole argument about Sancho only being truly effective on the counter and being predicted to struggle against deep defenses. This might come as news to people who still equate Dortmund to the pressing and transition machine it was under Klopp, but they have not been a primarily counter based team for a long time now. Sure, they are able to punish harshly on the break due to the pace the attack has and enough players who are capable to play accurate long passes, but this is still not their dominant way to score goals. They averaged 60 % possession last season (level with Leipzig and behind Bayern in that regard in the league) and played the highest number of passes in the Bundesliga with around 620 per game (that is nearly 50 more than United averaged last season). These are not statistics for a counter team, but a team that is tasked in the majority of the games to be the active creator of the play and break open defenses.
Jadon Sancho has been extremely valuable in that regard. He is a creative player with good (but not outstanding) acceleration, exceptional close control, skilled in 1v1 and has a very good eye for opened passing and running paths. Another often overlooked part of his ability to create space is his very smart off the ball movement which has improved greatly in his time with Dortmund (I believe that he learned a lot in that regard from Reus, who is a master in that). He often makes (or fakes) runs that demand the attention of defenders which open options for his teammates, the largest benefector of this being Haaland who directly moves in that space to create danger.
In short, Sanchos entire skill set is made for breaking up defensive lines.