Nest tags by using a slash, like #Novel/chapter/1 — this means #Novel will be the parent tag in the sidebar, and it will have a disclosure triangle. Tap or click it to reveal its child tag of #1 (and #2, and #3, etc.)
Infinitely nest tags by using more slashes, like #Journal/2017/04/06. This is a great way to create, say, a journal with a parent tag of Journal, a child tag of 2017, then a sub-child tag of 04, and so on
here it is: https://blog.bear-writer.com/organize-notes-with-tags-and-infinite-nested-tags-c42b02b2c0d7