I believe this is what Roam[1] is kind of built for, whether they intended it or not. I haven't used it but was inspired to take another look at it after this article.

[1] https://roamresearch.com/

There are some links here, from purpose-built to a post on using Trello for it: https://zettelkasten.de/tools/

Take a look at Zim wiki: https://zim-wiki.org/

For tasks, I use TickTick. I know it works for Windows, MacOS, and Android and it has been a godsend for me. The premmium features seem nice and I'd maybe one day pay for them but it's totally not necessary.

If you're looking for notetaking and documentation of stuff then I wouldn't really know where to start with that.

I am writing one as we speak (web based). Do you want a note when I get it out of the door?

Isn't any mindmap app (i use https://whimsical.com) a support for this theory of branching ideas ?