Hi there, @user03513.
I replicated your concern by adding a standard user and check if the User A can still access the Project B. I can see that the Project B is still visible to any standard user and there is no option to make this project only visible to a specific user.
For more information about the user access, you can check out this article: User types in QuickBooks Online.
I'll be here to help if you need anything else. Stay safe always.
