Hi Kevin,
Glad to provide my suggestion for you.
Indeed, we can choose how to open the files in SharePoint Online library, open in Online or open in desktop.
By default, in site collection, it will open in Office for Web when we click the document which the file can be opened in online. If we want to set only use “Open in App” for all the users, you can go to SharePoint Site Contents-> Library-> Library Settings-> Advanced Settings-> Opening Documents in the Browser-> Open in desktop client.
Note: This setting is only for library level, you need to set it manually for different libraries in a site collection.
If you have any further concern, please feel free to contact us.
Regards,
Ivy