How to turn off Details panel in SharePoint document library by default

To kick things off, hiding the Details pane for regular folders is a built-in option. Simply head to the document library, then click the gear icon and "Library Settings." Under the "General Settings," you'll find "Launch forms in a dialog?" - set that to "No."

Now, regarding document sets, the process differs slightly. First, navigate to the site collection housing your document library. Next, access "Site Settings" via the gear icon. From there, venture into "Site Collection Administration" and click "Site collection features." You'll want to deactivate the "Document Set versions" and "Document Sets" features.

However, a caveat - disabling "Document Sets" removes that functionality entirely. If you still require document sets, an alternate path would be customizing the behavior. That would involve utilizing SharePoint Designer or enlisting a SharePoint developer for a custom solution.

Circling back to regular folders, once you toggle that "Launch forms in a dialog?" setting, the Details pane should stay closed when users browse folders in that library.

As for document sets, deactivating those two features I mentioned earlier should prevent the Details pane from automatically opening. Just keep in mind the tradeoff of losing document set capabilities with that approach.