The movie uses two distinct timelines that eventually converge:

Christopher Nolan's 2000 film Memento is a landmark of neo-noir psychological thrillers, famous for its unique structure that mimics the protagonist's anterograde amnesia. The Narrative Structure

Told in reverse chronological order , these scenes represent Leonard’s subjective experience and provide the audience with the same lack of context he faces.

The film's complexity often leads to deep analysis of its ending and hidden details: A Deep Dive Into Christopher Nolan's MEMENTO (2000)