iPhone 7 not charging, but headphone adapter works.

On the lightning port, there are at least four connecions. Two of them are for charging (positive, negative). The other two relate to data transfer. If your headphones work, you should also be able to connect the phone to your computer and see it (but not charge it at the same time). Just to help you understand how things work, the charging aspect of your phone, consists of the charging port, and a charge controller chip on and a flexible, secondary, motherboard-like part that is responsible for charging, speakers, microphone and some other sensors. This means that the fault could either be the charging port, or the charging controller chip. What ever the case might be, you will need to replace the secondary motherboard. It costs about 6-7$ in total for the part ( https://www.ebay.com/itm/For-iPhone-7-4-... ). However, getting someone to fix it should cost about 80$. It is a mediocre level of a repair. I recomend at least going to a shop just to make sure that this is the problem.