John 21:2 leaves a clue as to who wrote the fourth gospel.
Jhn 21:2 There were together Simon Peter, and Thomas called Didymus, and Nathanael of Cana in Galilee, and the [sons] of Zebedee, and two other of his disciples.
If John wrote the fourth gospel, why would he have included “and two other of his disciples”? There would be no reason to mention that since neither of these two are ever mentioned again in the chapter. Simon Peter, Thomas, Nathanael, and the sons of Zebedee are mentioned because they are the apostles. (If Nathanael was also Bartholomew.)
If TDWJL was the author, the he would be one of the two other disciples. He recognized the apostles by name and the non-apostles anonymously.
https://debatingchristianity.com/forum/viewtopic.php?p=959548#p959548