You can change the search settings to limit which apps and results appear while you're searching. To do this:

Go to Settings > Siri & Search. Scroll down and select an app. Then select Show in Search, Suggest Shortcuts, or Show Siri Suggestions to allow or prevent results and shortcut suggestions from showing up. When you turn these features off, you'll have the option to select Show App. Turn this off to prevent the app from appearing in your search results.

You can also enable Suggestions on Lock Screen to prevent an app from appearing on the Lock screen as a Siri suggestion. Learn more about Siri suggestions.