4... Nc6 vs. 4... Qb6 in the French Advance

Yes, you are correct. I looked at some other forums on the internet and also had a quick look into the database and Qb6 first seems to be more accurate if Black wants to reduce White's options e.g. preventing the Kupreichik with Be3 as you mentioned in OP.

Most of the time when I encountered this in my OTB tournaments people only use this move order to play Bd7-b5. I never saw people play Qb6 first just to transpose back to the mainline with Nc6. Most people play Nc6, Qb6 first.

A friend of mine from the chess club plays this on a regular basis which lead to me looking into this line a bit more. I like Be2 and if Black plays Bb5 you play c4. You only need to study a few lines after which I think the position is in my opinion easier to play for White.

The question is now whether this is relevant from a practical point of view. Most people will play Nc6 anyway. But of course that doesn't mean that this never comes up.

In this case I think it isn't that bad because the french advance can be learnt by knowing some fundamental ideas like the importants of the d4 and e5-squares which don't have to necessarily be controlled by pawns.

This article on the french advance mentions this at the beginning: http://charlottechesscenter.blogspot.com/2017/11/opening-preparation-beating-french-with.html

In general, if you know the most important strategic and tactical ideas of the french advance you will always be able to come up with good moves even if you haven't prepared against a certain move order. If you learn to understand when and why to take away from the center with dxc5 you will easily navigate through the countless of move orders black can play at you (dxc5 is necessary in some positions to prevent your d4-pawn from becoming a weakness ; never allow cxd4 followed by Nf5 if there is no concrete reason for it otherwise you might just lose the d4-pawn)

For common tactical patterns in the advance I can recommend IM Lopez's 7part series on common tactics in the french advance. He covers tactics but also covers the strategic ideas as often they are strongly connected. Note that the specific more orders aren't that important in the video. It's rather the pattern you should remember. Here is the link to the playlist:

https://www.youtube.com/watch?v=-RVul5235f8&list=PLrTLUmPNfTCLtIc4vfBP_2is3E-7MLHAK

Regardless of whether you play the Kupreichik or a normal French advanced it is good to study the games of experts of the French Advanced to familiarize yourself with the most common middlegame idea. Examples for players that are relevant for the FA are Sveshnikov, Nimzowitsch, Grischuk and Shirov.

The move orders in their games are not that important try to focus on how they treat the middlegame or endgame. Full games are also good to get a full picture of how advantages get converted. You can also study the losses by these players to understand what could go wrong. If you have an annotated games collection with french advanced games than even better, unfortunately I don't know of any. Annotated games are scattered over many books.

This way you gain an understanding of the structure in general regardless of whether you specialize in certain subvariations or not.

Notice also that 4...Bd7 is another move which renders the Kupreichik as harmless.

1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Bd7 5.Be3 Qb6 6.Qd2 (6.Qc2?! Nc6 and the queen is exposed there) 6...Bb5 and Black has succeeded in exchanging the "French" bishop without making any serious concession.