Thank you for posting this in Microsoft Q&A.
In this situation you can make use of conditional access policies in Entra ID.
- Configure policy and in users step you can select "all users" and then you can exclude only one user to whom you want to give access to particular application.
- In next step select all application and exclude only one application that you wanted to give access to user.
- In the next step you can conditions if needed.
- Or you can move to next step to Access controls and select block access.
This will allow only one user to this application and this user will not be able to access any other apps.
Let me know if you have any further questions.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.