The one undeniable fact is that Jay knew where Hei's car was, so there is no denying he was involved in her death.

There is enough evidence putting Jay and Adnan together that afternoon, that I think they were both involved, but they killed her later that evening, not right after school as the defense argues.

I think it was a prisoners dilemma between the two of them. Because Jay spoke first and was able to get immunity for testifying, Adnan's only choice was to deny and hope to win at trial. Jay (eventually) crafted a story that fit enough of the evidence to blame Adnan, leaving Adnand with an inability to tell the truth without implicating himself, so Adnan has to keep up the party line that he is innocent, while Jay can make up anything he wants. They are both guilty and they are both liars, but Jay spoke first so he is the one that is free and Adnan is the one behind bars.

The true moral of Serial is, if you ever break the law with a group of other people, always be the first to come forward. You can say whatever you want at that point and get away with it.