So each individual guy knowing what he's supposed to do. Same as every position group. Never hear about chemistry among linebackers, receivers, secondary.
IDK. Never understood it. Still don't. Bottom line, this position group is the one I'm least likely to worry about as a fan. There has been perseverance due to injuries and other adversity. They have managed to still have success running the ball and pass pro despite the rotations.
If everyone can do their job it should be enough but there are subtle chemistry-ish things that make a real difference on the oline.
I played tight end so I kinda know how the fat guys work. Like someone else noted, it plays a role with double teams. Even when two tackles were equally capable getting movement, I preferred working with a guy who would get a little lift on the DE, even if that meant there was less movement b/c I could get into the defender better and drive. Some guys wanted the opposite, and would bitch if there was lift and little movement, but the lift fit my skill set. Maybe that's more of a preference than chemistry, but little preferences like this get accommodated over time within a the starting unit, and that's probably something that falls under the chemistry thing everyone talks about
Knowing when you can leave the double and get to the second level. A better lineman lets you come off faster, but even between comparable players, you often have a better feel with one. You both know when you need a second more or when it’s time to go.
Pass protection works the same way. If I stayed in to protect and had outside responsibility with a 7-tech inside, I’d get into my drop with my insdie arm extended to the DE's outside shoulder to slow him down a tad or change his path a bit. It allowed the tackle to play more straight up rather than overset which could expose him inside if the DE crossed his face. Presnap understanding of how I played allowed him to adjust and increase the odds of getting a good rep.
Presnap communication is easier some some guys, sometimes you know to widen or tighten splits based on what needs to get done and who is next to you, etc. Just a bunch of minor shit that adds up.
Other position groups depend more on chemistry across positions than within their own. QB to receiver is obvious, but linebacker with DL may be a bit less obvious or important. A blitzing LBer knows which linemen will sacrifice his own stats to make sure he clears the gap, and that may play a role in the LBers timing or path to QB.