I've tried several different ways to plan/outline my story. The first was a single Google Doc filled with a bunch of nonsensical ramblings; random ideas that I slapped onto the page here and there with little sense of organization. I listed out key characters, important plot points, lines I thought of that I wanted to use later, and just about anything I could think of that I wanted to remember. But as you can imagine, that jumbled mess of words on a page was hard to use reliably to work on a story. It helped me a little bit, but not enough.
So instead, I took to creating a proper timeline of events from start to finish. In my story, Micah Ever After, a timeline is essential to help me keep track of exactly what happened when, since so little is shown to the reader because of the protagonist's limited perspective. As one of its tags suggests, the past plays a big role in the events the protagonist experiences, so making sure I understand those events and their relation to the overall timeline is important. The timeline is huge, but organized in a way that lets me know where to look if I need to remember a detail I've forgotten or want to refresh certain information in my mind.
To help me even more, I also have a private discord server where I log background information that doesn't necessarily need to go into the timeline, such as character backstories, details about important events, and also potential ideas. This and the timeline together are what I currently use because they're much more organized, but I do refer back to the that jumbled doc every so often just in case.