We had an interesting support question on how to see internally shared docs.
Under Drive Audit to see all documents that are shared internally simple click on the link that says ‘Open to other users of this domain’
However, there is more to this answer than a single click…
These documents may contain some that are shared in from the outside, to eliminate these simply click on the ‘X’ beside the ‘In’ filter above the results. This results in documents shared ‘internally only’.
Documents shared externally or Public, may also be shared internally, do you want to consider these also? Then see note 3.
You can use powerful Reg. Ex. to explore the metadata – see this useful guide. (Hint, try an expression like this [a-zA-Z0-9-]+@(your-domain\.com) for General Search under Editors, eliminate external shares in and repeat for Viewers. This will show all internally owned files with local editors or viewers, even those shared externally or Public)