Trillium Notes is another one that allows for the same node to be available in multiple branches. It's pretty good - but the focus on rich-text content primarily became frustrating for me and got in my way and I started looking at other options. I then found exporting/migrating my data with those concepts was challenging, since it ends up being proprietary with no easy export option - at least no easy export that then cleanly works in other products.
The markdown world has some contenders with embeddable links that allow you to include notes from multiple locations (like an iframe, basically) - Obsidian.md being one such example.
WAAAY back in the 1980s/early 90s there was a file manager called "pathminder" or PMV...
It was THE best file manager ever. I could browse my entire hard drive from memory alone via WASD type input and I would know exactly where I was just from memory and key presses...
If you ever played the game "The Bards Tale" there was a version, maybe 3 or such, where you could recall exactly the steps taken to get to a specific dungeon and could type that out very fast and get there via just recalling the steps it took to get there....
PMV was like that.... you knew that [Directory] was a series of steps in PMV which you could quickly type out.
An Outliner, Editor, IDE and PIM written in Python. Cross-platform too.
https://leoeditor.com/ the homepage of the program, with screenshots.
https://github.com/leo-editor/leo-editor
There's a Wikipedia article about it too even.
https://en.wikipedia.org/wiki/Leo_(text_editor)
Leo means Leonine Editor with Outlines.