Having announced a big new, more accessible (e.g. for blind users) version of OneNote back in May, the revised UWP application is now rolling out to mobile users, at least on the Fast ring - and production users in due course. Much of the change is on the desktop, of course, with graphical layout tweaks, but there's still something of a facelift for mobile, as shown below.

From the original Microsoft post:

Today, we are excited to announce that we updated the design of OneNote for Windows 10, Mac, iOS, Android and OneNote Online. These design features, rolling out over the coming weeks, include three key areas of focus:

• Enhancing usability for those who use assistive technologies.
• Simplifying navigation controls.
• Creating consistency across devices.

Simplified navigation

We worked with users to learn how we could improve the navigation layout—especially for larger notebooks with more sections. Now, the navigation controls are all in one area on the left-hand side of the app. This allows users to easily switch between their notes and dramatically improves usability with assistive technologies. With the new consolidated and simplified design, screen readers can easily navigate through the app to help those with disabilities. In addition, content is front and center—helping students to focus and avoid distractions.

The hierarchical layout above is transformed into a more simplistic nested set of panes for mobile, even in landscape and on larger phone screens. This isn't as 'clever' as in some other UWP applications, but if the driving force here was accessibility then I can understand - showing 'one pane at a time' is one pane at a time to 'narrate' for blind users.

Some before/after screenshots of the previous/new version of OneNote UWP:

Previously navigation between notebooks was via a hamburger menu (and no, I don't know why my recently synced notebooks have warning icons either!); (right) the notebooks are now just one step back from the list of 'sections'.

Previously, opening a workbook showed sections as tabs, with entries peeking through in a scrollable list; (right) the sections are now shown as the 'next' list...

...and you have to tap through again to get to titles of entries, and then again to get to actual content. All super hierarchical and super-accessible, though the deviation from what happens elsewhere in Windows 10 is notable.

You can update this in the Store in the usual way once its rollout has reached your device/ring/country.