Chapters first came about back when books were published n the newspaper, so you needed a section that ended with something exciting to entice people to buy the paper again to get the rest of the story. There's nothing that says you HAVE to break the novel up, but we've gotten used to the format.
Traditionally...
Scenes usually break when the story needs to jump ahead or to another location. You don't want to show the boring travel stuff, so you skip ahead to the good stuff. They also break when you change POVs.
Chapters usually break into chunks that show something trying to be achieved, and end with the resolution of that goal -- win or lose. Like rungs on a ladder heading to a big finish. Often, an event will span multiple chapters as it escalates. Chapters can also break to change POVs as well if you do one chapter per character. There's often a thematic element typing a chapter together, but not always.
Parts usually separate time jumps, different characters, or larger thematic elements of a story. Often there are things that tie all the events of one part together in some way.
Series or volumes usually separate individual stories set in the same world with the same characters. A series is different from a trilogy, (or duology, quartet, etc) in that series books are each self contained stories with similar characters. A trilogy type is one story spread out over multiple books, even if each book has a complete story in and of itself. All the parts are connected to tell a larger tale.
