By default, Windows will use the index when searching to give you faster search results. The search index only include your selected locations. These locations can be filtered for what file types (extensions), file properties, and file contents you want indexed.
The index uses the Windows Search service and runs as the Searchindexer.exe process in the background. The index will automatically rebuild and update for changes made to the included locations since the last index rebuild to increase search result accuracy. By default, the indexing speed is reduced due to user activity. If the computer is running idle, then the indexing speed will go back to full speed to finish indexing faster.
If you are getting search results that are missing, no longer valid, or incorrect, then the index may not be up to date. You can manually rebuild the index to force it to update now.
This tutorial will show you how to manually delete and rebuild the search index for accurate search results in Windows 10.