Apart from the passion there are very limited points of contact between John and the others. Most of the "more famous" miracles or parables are in either John or the others (either all of them, or some of them), but almost never in both. While Matthew and Luke have "unique" episodes, they also have a lot of shared content between themselves and Mark.

Plus, when they talk about the same topic, the synoptic gospels have entire sentences that are basically the same, word by word.

For a comparison table see https://www.ammannato.it/vangelo-di-vangeli/appendice/cronol....

(Just someone who had a very good religion teacher in middle school).