How can I trade iPhone in Apple Store?

Apple has a trade-in program. According to their site, an iPhone 8 Plus has an "estimated trade-in value" of "up to $80 [USD]."

I don't see any 11 Pros for sale on Apple's U.S. site, but they seem to have 12 Pros in the Certified Refurbished store on a fairly regular basis. There are two 128 GB iPhone 12 Pros there right now that are going for $589 [USD] (each).

Apple Trade In - Apple

Refurbished iPhone Deals - Apple

The iPhone 11 Pro is no longer available for purchase directly from Apple. However, you might still find it at some Apple-authorized resellers and wireless service providers.