When I tried to login using my non-admin user account, I can see two billing accounts: the one linking to the parent billing account, and another billing from child account itself. It’s normal to see two billing account? Because from your video, there is only one billing account which is the parent account (admin)
You can create your own billing account for the new user created. Its upto to the new user to properly manage and add the projects to the billing account they desire. The reason why Mattias didn’t show that feature is he did not add a new billing account for that user. If you just create a new user in gmail and go directly to the console when the admin has already added the new email, GCP does not ask you for adding your card. So you can directly add projects and you will see them on the billing console. But you can add your card for the new account and have separate billing accounts created.So it is fine when you see two or more billing accounts.