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Looking for OneNote Alternative
(feddit.de)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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As your description is rather short and does not really restrict the "recommendation space", I'll start the round of recommendations with Joplin
I used Joplin on Windows and iPhone - syncing (encrypted) using a OneDrive account. So far working well.