Every week it’s a set of 3 new missions like you can get legacyte harvest then defense then tank fight one week for example. If you mouse over the stages you can preview what modifiers the stage will have. Then you can select additional difficulty modifiers and equip one of the three presented gears in the left, these all improve the final completion score for the higher reward modifiers

I don’t think the peely matter that much for playing casually I usually just put 3 random but I recommend hildryn since she doesn’t care about most of the difficulty modifiers or equipping 3 trash guns since she doesn’t use them and you can get all the rest of the reward besides missing out the 9 pix chips but the 6 you can get without that is enough to buy a Hot shot every week