How to filter single posts view to allowed posts only?

If you give that user a specific “user group”, then you check that user group in the “access” area while editing the post. The post will only be visible to anyone who is in that user group.

Alternatively, you can create a specific category for that post or posts like it, and then check off that user group in the category edit area. Then, only those who are in that user group will see anything that is given that category.

Similarly, you can lock or restrict a media file by categorizing media and giving that/those categories a user access code.