Dear friend, all true religions are aspects of each other. They vary because the audience has differing capacities, needs and degrees of social development, that said, Jesus obviously understood that He needed to bring aspects of past religions into His own Revelation to substantiate and legitimise the ongoing process of G.d Educating His Creation. This is also a deeply ingrain aspect of Hebrew, Jewish culture, they embed the past in the present as though they are part of the same reality.
Christianity is derived from the NT and no other Scripture. That Christ should choose to reference the OT is to show that G.d is active in every age and some truths are eternal.
As for countless prophecies being fulfilled in Jesus Christ, name three.
With the advent of Jesus all previous scriptural teachings were secondary to every Teaching and Pronouncement made by Christ.
