Although I’m a fan of the Marvel movie universe, I don’t know all the references that connect the various films. That’s why I never caught on to “on your left” until I coincidentally saw the two key scenes recently. I’m talking about these scenes.
The introduction is when Steve Rogers continually laps Sam Wilson during a morning run, saying “on your left” as he passes by.
Then in Endgame as a beaten Captain America gets ready for one more round with Thanos and his army, it’s Steve who hears “on your left” as spoken by Sam as the revived heroes start portaling in to join Cap for an epic sequence.