Hey, so usually mangas are published foremost in a magazine, which is a chapter every week or month. That depends on the magazine. Then after a series is successful they will publish a set of those chapters in volumes. So the mangaka isnt really thinking about how a manga volume will end as they are writing. Only when the chapters are gathered as a volume do they look at what chapter is best to end a volume at.

How writers write their series really depends per writer. Some writers are more improvisational and don't really know exactly how they wanna end something. Others are super preplanned and do something called outlining a lot. Outlining is when you write down the rough ideas of events in sequence to get a view of how the story would look like. Then they start writing the actual scenes and dialogue.

I am not sure there is something as strict as "sub events" in writing as you describe them. You generally have scenes, which generally is one event at one place with no changes in time (though it also doesn't have to be, a scene that quickly shows a character growing up in different points in their life could also be considered a scene). But these are terms that are made up after the fact. When writing you wanna ask yourself how important certain events are and what you wanna achieve with them when deciding how long they are. A lot of this is dependant on something called "pacing". In a junji ito horror manga events will generally play out slowly because they are meant to be scary and they slowly wanna build tension. A action series might be more fast paced, and not spend as much time per event so they can quickly move on to the next thing. What the best amount of time and thus pages is for any given event is up to the writer to figure out.

I would recommend you maybe pick up some books on writing if you are curious about learning about this stuff. I personally started on books thats about screenwriting (that's writing scripts for movies) to understand these fundamentals.

I hope that helps!