Shout out for Joplin [1] for digital note-taking - coupled with Nextcloud, I have a self-hosted note repository that syncs across my mobile, laptop and desktop. Interface is clean, notes are in Markdown format, and notes can be cross-linked and organised into notebooks. And it's OSS.
I could be wrong, but I think as long as you have access to the same Dropbox account, you can sync up as many devices as you want. No nextcloud integration required.
Yes, Nextcloud is not required - I'm saying that having that gives me self-hosted synchronisation. You can indeed use Dropbox; there are other options too.
Not affiliated, just a fan!
[1]https://joplinapp.org/