How expedition explosion setup works?

I put those green circle on top of the chests but sometimes they are not excavated. How does the league mechanic work please?

When you say on top of the chests, do you mean the boxes on the ground that say "Unearthed Remnant"? Those are not the chests, those are things that make the fight more difficult but make the rewards better. The chests are buried and have the signs with the picture of a chest on them. Some of them have that yellow outline. I don't know why. I assume those are better? but who knows, some info on that would be nice. But anyways, make sure the signs with the pictures of the chests are in your green circles.

Ah ok . thank you very much for the information! Just those boxes looked like chests to me :D

these things are called Remnants. Additional info on mechanics from Chris:

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When the explosion occurs that hits that Remnant, they apply to all monsters and chests unearthed by that explosion and all subsequent ones.