I was going to comment to recommend orgmode too. Like you said, the org files are plain text, they can be opened by practically anything, and can be saved practically anywhere.
I use Orgzly on Android, which syncs manually to DropBox. A free dropbox 2GB space is going to store a lot of orgmode files! I edit mostly through Orgzly, although I do use the orgmode extension in Visual Studio Code to edit on my laptop.
I guess the real answer is "a folder of text files you sync." You don't have to use Orgmode or emacs at all to get a good chunk of the benefit here.
True, you can't do rich text or media, but it turns out I almost never even WANT to do that.
Anyway, I was doing sync'd text with a Mac tool called Notational Velocity for years before I got sucked into OrgMode (emacs' Deft mode is a workalike). Point any iOS or Android editor at a Dropbox folder and you've got mobile sync on the cheap with versioned backup and more or less ever present and completely open data that works on any platform.
OneNote and EN may have some shiny features, but NOTHING they do is worth locking your data up.
> True, you can't do rich text or media, but it turns out I almost never even WANT to do that.
And the times that you do, you can use Markdown and Pandoc to convert it. And mdfind/spotlight will index and search it just fine, as will any other search tool you wish to use.
I lament some of my coworkers inability to see the utility in text+markdown+pandoc or even just a plain old wiki like Dokuwiki. I've lost count of the number of times I've had to wait a day for someone to dig out an excel spreadsheet to email me an immediately-out-of-date copy.
While I will joke about how people would find it so much easier if they just did everything I said, I do wonder why so many people keep themselves permanently handicapped by their choice of information storage and sharing.
Well, what is easy and intuitive and quick for you might not be for other folks, so there's that.
The immediate staleness of traditional Office docs is a downside, but the upside is that you have a copy you control. It falls down a little on collaboration, though.
I'm 100% done trusting proprietary formats for this data. My OrgMode files are all plain text, so they can be read on any platform.