Can I search inside just one folder?

Yes. Open the folder and tap the magnifying glass in its header. The screen is titled with the folder's name so it is clear where you are looking, and results come only from the lists inside it.

Searching from the home screen still searches everything, wherever it is filed.

The search screen showing a result labelled with the list it belongs to — Every result says which list it came from, so a folder’s worth of lists stays legible.

Still stuck? Email me — a person reads every one.