Simple. the receivers feet were in bounds, and he caught the ball when the ball was actually beyond the end zone line (in the back of the end zone). It’s a catch. Happens all the time. For that matter - the ball can be out of bounds - but it’s still a catch if the players feet are in bounds. See the rule inconsistency?
The receiver was interfered with whilst the pigskin was in the air and body blocked in a way by the DB toward the back boundary line by the time the ball was intercepted; however, the contact began several feet earlier in his route and it can't be known with certainty that he could NOT have made a play on the ball. I'd guess the chances were not better than ~25%, but the rules of governing non-catchable official judgement exclude any possibility.
Plainly that would not accurately reflect what occurred on the play.
The ball, just like a player, is in bounds until it/he touches the ground out of bounds. Why is that challenging to understand?