I'm very interested in OneNote alternatives. I've been using OneNote for longer than I've disliked Microsoft... and I still think it's a good app. Pretty much the only thing I don't like about OneNote is the increasingly close integration with Windows accounts. I'd rather my notes were not scanned and uploaded and processed by a US mega-corp...
Anyway, people keep suggesting stuff like Joplin. But that isn't even vaguely close to OneNote in terms of stylus and inking. For me, inking is the main core feature. And so in terms of alternatives, xournal++ is a closer fit. (xournal++ has essentially no organisational structure for notes; but it is really great for inking.)
What I'd really like is basically the organisational structure of Joplin (or whatever other alternative) + the inking power of xournal++. And ideally being able to import my vast amounts of handwritten OneNote work! -- But that last bit is probably too much to ask.