After playing 5.Qb6 which I believe is the final move of the Advance variation of the French, my opponent played 6.a4. I followed with cxd4 and them by cxd4 as well. After that I couldn't remember the best theoretical move so I just played 7.Bd7, as I often find it hard to activate it later on when opening with the French.
[fen ""] 1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6 5. Nf3 Qb6 6. a4 cxd4 7. cxd4
I analysed the game with Stockfish and looked at the database on 365Chess, and saw that the most played move in this position was 7.Bb4+ which makes sense: developing the bishop with tempo, adding some pressure, without real risk before continuing development. However I let Stockfish run until depth 42 and these were the top 3 moves:
Bb4+ apparently isn't such a good move in this position. Any ideas why ? As I've said, I don't see the problem with playing 7.Bb4+ then 8.Nge7 for example. I acknowledge that it lets the opponent develop their knight on c3 but don't have the chess level to know why it's and advantage for them more than it is for me apparently.