Below is a graphic representation of the number of times Jesus quotes the Old Testament in the New Testament. There are some interesting patterns here.
Firstly, note how the the Gospel of Matthew is the most quote heavy of the four all and that there’s considerable focus on the Torah, the first five books of the Old Testament. This supports the common claim that his gospel is the most Torah focussed.
The Gospel of Mark also puts substantial weight on the Torah, and it’s the only gospel that references Ezekiel and Joshua. For such a brief and action focussed gospel its surprising how many quotes it references.
On the flip side, note how the Gospel of John, the last to be written, is the least intertextual of them all, only referencing Psalms and Isaiah.
Studying such interconnections between these books can help us to get a better feel for the concerns and intentions of the authors of each gospel.