I've read that you want to make pawn breaks by advancing your pawn on the file next to your most advanced pawn in these closed centre positions. What exactly is the reason behind this. Is it just to open up the files for the rooks and why specifically with this pawn? I'm finding this topic incredibly confusing so any help will be appreciated.
Opening a file for your rooks is one good reason to play a pawn break. Although, you should make sure that opening that file doesn't help your opponent's pieces more than it helps yours. As for breaking through near your most advanced pawn, this isn't really a rule that has to be followed closely. But the idea behind it is that once your advanced pawn becomes unblocked, it can push forward into the enemy position. Maybe this will allow it to become a passed pawn. Or, it could open lines for the rest of your pieces to break through. This may be more dangerous for your opponent than if you had broken through further back on the board.
Basic rule is keep your pawns together or near if possible, especially towards the endgame. If you have an advanced pawn, and another one next door, the obvious thing is bring it up. Notice that with two pawns together on the 6th rank, they are very hard to keep at least one of them from queening.