How is a message's protocol determined in wireshark?

I am new to wireshark. I started watching one training video but it was long and I am looking for specific answers to questions to help in my coding job.

How is a message packet’s protocol determined in wireshark? I have a .pcapng file I have been looking at and at first it seemed that the first three hex digits were the determining factor because they seemed to be unique to a protocol. But this is not the case. Instead they seem to be part of the destnation address.

Thanks in advance.

Also, just to be sure: the hexidesimal representation in teh third frame window represents the whole package without anything added or taken away, right? Is this a correct assumption?