You can search Notes in specific Folders in the following ways.
<Desktop>
1. Enter keywords in search box.
2. Click a Folder and you can refine the search target to the designated Folder.
<Mobile>
1. Tap “Magnifying glass” icon on the upper left.
2. Enter keywords in search box.
3. Tap “Filter by Folder”.
4. Tap a Folder and you can refine the search target to the designated Folder.