The film structure is that of a Raymond Chandler-style detective noir
The Big Lebowski has a very unusual narrative structure from first glance. It’s not quite a buddy movie, it’s not quite a comedy of errors, and there’s all this very strange espionage going on by increasingly shady and bizarre characters double-crossing each other. It makes a lot more sense when you realize the film is essentially a classic detective film, except the protagonists are under-qualified, accidental detectives. This lends a lot of the humor to the movie, as the protagonists are completely over their head and have no idea how to navigate the machinations going on around them.
It has all the tropes. For instance, like detective movie protagonists, the Dude appears in literally every scene of the movie, just occasionally shoved so far in the background that he only “appears” as the van he is in through the window at the diner where the nihilists are ordering pancakes. The whole movie takes place in the vicinity of wherever he happens to be. There’s even a scene where the Dude is accosted by a detective that acts as if he is playing some sort of 4-D chess spy game, rather than just constantly blundering into wherever the action happens to be. It’s an incredibly clever reversal of expectations that even devoted fans of the film may miss.