how to use the coins?

[Medivh]

I get some type of special coins but not sure how to use them

[Watermelons]

Lunar Coins, they're persistent across your runs, you spend them for Lunar items. They can show up in these white pods with a blue light randomly in the game and contain blue Lunar items, which follow a theme of having a powerful positive effect and negative effect. You can find the "Newt Altar', which is a blue frog statue that will guarantee a Blue Portal showing up when you successfully charge up that level's Teleporter event; using this blue portal will send you to a shop where you can spend more Lunar items at a display table that you can reroll, as well as a new character, and guaranteeing the next map upon leaving the bazaar. There are also 2 displayed items of green and red rarity that you can donate items of the requested rarity to receive. And finally, this bazaar also has a hidden portal access to a hidden challenge map for more items.
In short, find things that ask for the currency, there aren't many. Lunar Pods, Lunar items displayed in the shop as well as rerolling those displayed. The map selection in the bazaar. And also the rare "Shrine of Order" that can spawn randomly ingame, though I *highly* don't recommend doing that lmao, unless you're ready to really jumble up your item inventory.