Played the French. Got destroyed. What did I do wrong?

You let your opponent have all the piece activity and locked your position down and then eventually was able to convert that to a material advantage. You seemed to play on early on, perhaps you might have been better had you kept developing your position instead of going after the c2 pawn.
It looks like their pawn sac (letting you take the pawn) was successful, as they had a gigantic lead in development.
Generally, the French is not a fantastic opening (white wins about 40% of the time.) This may have been a large factor because you seem to have misunderstood the strategy. I would not have recommended playing your moves after 8. a4. It looks like you seem to have memorized a variety of certain openings derived from the French, but do not know the core strategical aspects of the opening. I would recommend studying master games and videos concerning the French, and keeping a close eye on development and direction of the pieces.

8.a4 is a line created by Smyslov and championed by Fischer. The idea is to play an early Ba3 and dominate the dark squares. A famous game in that line is Fischer-Larsen, 1st match game, Denver, 1971. White chose another approach, a pawn sac I'm not familiar with. I'm not crazy about your 11....Qxc2. Part of the theory in the Winawer French (and all French variations, for that matter) is to control the c-file. With that in mind, I'd recommend 11....Bd7, following up with Rc8 and taking the pawn later. The c-pawn isn't going to run away and you are completing your q-side development. 13....b6 might have been better, protecting the b-pawn so you could answer Bb5+ with Bd7 and it is tougher for white to win back his material. After 15.Bb5+ your pieces were in no position to resist. Hope this helps.

I think the Winawer is a very variation rich line. I play the French myself and avoid it with both colours. The a4 line where you go 'huh' was populised ....or a earlier move, by Fischer who has a couple of Winawer lines in My 60 Memorable Games - his record against this line was not too good by the way.
You might just need more practice and study with it.
Looking at the choice of capturing the hanging pawn, (a critical decision), I think seeking the trade of queens was slightly preferable due to the lack of space (and underdevelopment); 11... Nxc2 12. Rb1 a5 13.Bd2 (13. Bb5+ Bd7 14. Bxd7+ Kxd7 15. Bb2) Na3 (13. ... Nb4 14. Bxb4 axb4 15. Rxb4 Bd7) 14. Rc1 Nc4 15.Ng5 Nf5 looks more unclear.
However, I think you had to be more pro-active in extricating the knight in the variation you chose; so, to that end, maybe 13... Nc6 was a better choice, since it adds extra defence to the b4 flight square, still hits the d4 pawn, and covers any bishop checks. Then if it were to continue, similiarly, 14. Rd1 you can hang onto the pawn with immediate b6, a5, ideas.
18.h3? failed to exploit the knights poor position, since 18. Rbc1 Na3 19. Rc7 Nxb5 20. axb5 threatens the back rank (hitting the bishop); so, there was the chance to resolve that piece via the immediate 18.h3? Na3 19. Rb3 Nxb5 slowing down the rook invasion.

The following forum thread details the ideas/goals for the a4 move:
French Defence/ Winawer Variation
https://www.chess.com/forum/view/chess-openings/french-defence-winawer-variation2
I think the biggest mistake you made was not developing your light squared bishop. Even moving it a modest one square to d7 would have helped your game a lot. As a result your rook on a8 was stuck there, blocked by the bishop, and unable to contest the white rooks on the c file. The white rooks were able to dominate the c file and basically paralyze your position. Your rook on h8 was also cut off from the action, because the knight blocked it. I think at some point around move 22 you should have made an ...f6 pawn break. That would at least have given your knight on g8 some scope, and allowed the rook on h8 to come towards the center. Generally speaking your rooks seemed to be uncoordinated throughout the game and unable to challenge the white rooks.
7)...Qc7 was probably too passive. ...Qa5 is more active and forces White to defend the c3 pawn with either Qd2 or Bd2
Don't get discouraged though the WInawer French is a very complex and "fiddly" opening and I almost gave up on it many times! It can be hard to believe that the "solid" French can produce such a tactical beast.
I don't know this line too well, but to me it looks that white does not have an edge after the pawn sac. White does have some compensation but I would take black every day. Need to come up with a good plan to free your knight without dropping a pawn. Would probably go for 13 ...Nc6 instead of ...Nf5
That way you put pressure on d4 and after Rd1 your knights could cooperate on b4 if necessary. Stay on c2 until it's necessary to move that knight out, until then you can develop maybe O-O and prepare to play for Bd7 without dropping the b-pawn. I prefer black already
I've been toying with playing the French for a while and have started testing it out. I got destroyed in this game though...are there any obvious or thematic mistakes that you think I made? Thanks in advance for the help!