On a solo project, I write a journal, and start finding patterns in it. For e.g. if some parts are front-end related. i add some emoji, or colored text for this and it becomes a "topic". Over time I add Lists for TODO and TODO-later, where I move these items between. The simple act of having colors and graphics helps me keep a high level overview.

For teams, I am experimenting with Github issues. I have added labels for topics and urgency. I chose github, because (i) as a developer I spend a lot of my time there. (ii) I find its editor "quiet". google docs etc take time to load, I want a place to quickly jot down an idea to get it out of my head (iii) It keeps the project management alongside the code.