Hi @Nancy DEMERS,
Thank you for your reply.
Of course, you can set files on SharePoint to open with office apps by default. You can set this setting to apply to files in a document library or to files on all sites in a site collection.
1.Setting all files in a document library to open with office software by default. To set this up, make sure you have full control over the document library.
Please go to Site contents->click the document library which you want to set up ->Settings.
In Settings page, please select Advanced settings->Opening Documents in the Browser. Setting up to open in the client application.
2.Setting all files on all sites in a site collection to open with office apps by default. To set this up, make sure you are the site collection administrator.
Please go to Site contents->click the Site Settings->Site Collection Administration-> Go to top level site settings. (If you are already on the top-level site of the site collection, then you can just go to Site contents->click the Site Settings->Site Collection Administration).
Please go to Site Collection Administration->Site collection features->Active the "Open Documents in Client Applications by Default" feature.
If you can't see the Site collection features option, then you are not an administrator of the site collection. You can view or set up the site collection administrator based on this article:
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In my test, the .csv files will be opened in Excel by default.
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