How does a trade in work at the T-Mobile Costo kiosks?

I called the special Costco T-Mobile line about trading in my iPhone 13 Pro Max to upgrade towards a 17 Pro Max. They said that my offer from T-Mobile was $300 but that $170 would be deducted from my trade offer and applied to my account as bill credits. How exactly does that work? Do they just load $170 in statement credit in my account to use? Can I use that $170 towards a specific EIP on my account since there’s multiple lines on the same account? Is my next bill just automatically going to be $170 cheaper? And if my next bill is $170 cheaper, how exactly do I calculate that because my T-Mobile plan consists of my own line, my mom, my sister, and my nephew? Any clarification on this would be appreciated.