+6 and still a draw?

https://www.chess.com/game/live/82565604179

The engine here shows +6, but it looks like a draw to me...

But why play 5. .. Bc8 and not Na6 instead?

because ...Na6 frees the advance of the white b-pawn and gives additional options to white (even if not instantly available).

7.Bc7 here is a blunder, it plays into the white's hands to play c5. Black can surely do better. After the sacrifice by white, the engine says +1 so it doesn't seem to be working. Of course something like you did must be the answer, if there is one.

By the way @OP, if you let engine run it is not +6, but around +2.9 (it may be that it shows +6 for you because of low depth analysis), which still means winning, but who knows in these closed positions.

I agree that closed positions like this the engine has been known to overestimate certain factors (like material) and misevaluate due to the horizon effect. I originally thought this was winning for white, but admittingly, I'm not 100% certain now. I just thought it was winning, but sometimes things aren't so clear.

However, what would you recommend instead of 7...Bc7? I was just trying to maintain the blockade and keep things closed. The King is blockading the passed e6 pawn and I wanted to keep the Bishop on a6 if possible.

Well, the problem is that c5 doesn't work by itself, or at least not yet anyway. Waiting move that doesn't allow tactics like Bb7 or Bc8 and c5 might be a good try, but it doesn't seem to be working for now.
Though surely black has to be careful later on. I can see myself losing this of course and something like what you did seems like the only try for white to go for a win - he is not making too big of a risk because, as you stated, he is up by quite some material.

This is pretty interesting position by the way. It shows the power of rooks if the position opens up. So later on, even if black has the opportunity to win some small amount of material, he should try to avoid it if it opens a file.
For instance:

So the fact that he could get to even material doesn't help him with all those weaknesses, so he must avoid it at all costs. Seems to me that black can avoid it, but perhaps there is something different that white can do to make your plan better. I am just too weak to see it.

Plus it seems to me that the fact that white has such a terrible bishop is diminishing his play a lot. If it was a different color bishop, maybe he would be able to sacrifice it somehow to open files and that could be enough. At least he would have a different approach as an option.

What a silly cooperative line this is...

Can you show us your glorious plan after 7...Bb7, or 7...Bc8?

To the O.P: The online chess engine of this site is rubbish.

If you have used a good local engine, you could well verify your feel, which is absolutely true: White cannot break Black's fortress.

The engine gives exactly the same evaluation to almost any sane move by white, which is a very strong indication that no progress can be made.

Black's way of holding the position is actually very easy; You just set your pieces up like that, and then move ...Bc7-d8 until white either sacrifises the b- pawn (when you take with the c5 pawn and then plant the knight at c5) or he tries Ka3-a4, when you simply play ...Nb4, and all he can do is retreating.