Local search currently uses Google, mostly because it supports "natural language" queries and is more tolerant to ambiguous input. This makes the preset categories easier to define and also makes it easy for users to write free form queries (the current local search mechanism used on Sailfish OS).

I investigated how to use OSM for local search last time Google broke the local search API, but then got patches for using the new API from Geoff Kuenning and went with that.

I would like to eventually add OSM based local/POI search, but it will be quite a bit more complicated than using the simple Google API - but also much more powerful! It would probably work fine with a preset list containing relevant tags for search, but free form search would be most probably limited to advanced users knowing how the OSM tags work, given how the OSM data is structured.

As for the other search options, address search is using Nominatim (so it is based on OSM data) and Wikipedia search goes over Geonames.

Yep, the preset list could be defined like this.