How do you know when your chapter is finished?
Once you’re clear that all of your thoughts are out about the subject of the chapter and you’re ready to move to the next topic (or situation if it’s a novel).
How do you do when your book is finished?
It’s really good to outline before you write, or use bullet points. That way you have a reference to check off that you included all that you want to convey to your audience.
Keep in mind that once you’re finished and you read through your manuscript you may have an “aha moment” where you decide to add more information to your chapters. You also may decide to delete some things also.
I hope that answers your questions.