I can't even get a queue in to the expert mode and have been doing advanced. Solo queue, as I don't have a team. I have heard some of them are just queueing in group mode and then giving everything to one person, is that true?
#So, someone share their wisdom with me. How are people extracting with 15mil+?
bold thicket
left cove
I suspect they got the Jadecrest Runner rare event while doing Solo... apparently they can drop reds, and the event spawns like... 11 of them or so? So if you're solo and go around looting and getting multiple reds... It can add up to over 10 mil easy (esp since each red can be 1-5mil? per piece) I'm just waiting for RNG to bless me to test this out myself
It definitely involves reds, because even with an entire inventory of yellows you would barely break 10m (and RNG is not that kind, even on Team mode) and the only reliable way I know that involves spawning multiple reds is the Jadecrest Runner event.
dense gulch
Should be possible? since getting a full red would require at least a rat king, jade crest, or a super lucky rng in one run. Or someone willingly giving away their reds. I haven't encountered a kind soul yet 
left cove
Should be possible? since getting a full red would require at least a rat king, ...
Rat king isn't actually that profitable, you only have 3 chances to trade with it and I see players trading yellow 3 times and only getting a red back 1 time.
Though if you're in multiplayer, then different players can each trade 3 times and maybe give away any reds they get like you said... but that's not really proper gameplay anymore.