I've all ready got Sins of a Solar Empire on Gamestop App.

I would like a complete game, with all expansions + $10 discount (limited offer) https://www.sinsofasolarempire.com/store .

What is step by step process?

1) confusions around Trinity:
explain how Trinity functions please.
This adds content to even a new rebellion install?
Is it like : Trinity = expansion 1, Rebellion = expansion 2? 
They build on each other, or are they seperated, so that installing Trinity will not have an effect on a Rebellion install?

 2) Regarding the "$10 discount": 
Would I have to buy the "Sins of a Solar Empire: Rebellion Upgrade-" from Stardock.com store and this would give me a coupon to use on GameStopp App? Or does it only work on Steam?
I would have to buy trinity + rebellion on Steam to get the discount? 

 I would like a complete game, but if Trinity and rebellion are not interacting with each other, maybe just buy rebellion?