So to clarify, there are 2 main different types of Coins in Dokkan. These coins can be obtained by summoning on different types of banners and can be used to obtain different units. Let me start with the red coins. These coins can be obtained by summoning on Dokkan Fest banners like this one

And further more, if you get 400-500 coins you can purchase any dokkan fest unit featured on a banner or...you can purchase a Dokkan fest LR

This will include, at some point, LR Blue Vegito and LR Blue Gogeta that Global just got for the 5th year anniversary. Now the Gold coins are pretty simple, you summon on a banner like this.

Get 500 Gold coins and you can purchase any summonable LR say like LR Kale and Caulifla or TEQ Broly.