There is a setting in the app permissions that is typically enabled by default to allow the app to run in the background. You can disable it, but I believe it is a per app setting.

Alternatively, if you turn on battery saver, I believe that turns off background app usage.

On my pixel, if I type something in to the search bar so it displays search results (any results) and then scroll all the way to the bottom, there is a settings button I can click on that takes me to settings for the search. If I turn all the options off, then I get just my local apps in the results.