If you have an old iPhone can you trade it in to get a new iPhone or another device

depends on the device you want to trade in.. but in general, yes

Apple Trade In - Apple

As mentioned it depends on the model (and the condition) of your old device. Sometimes though you would get more selling it yourself than you do with trade-ins. Example...I sold my iPhone 11 Pro Max for $850. Apple and my carrier trade-in offers were far less so it wasn't worth it for me. Last year it was better to trade in my sons old iPhone SE (the original version) for his iPhone 11 than it would have been to sell it.