What can be best database schema for multiple project and one database?
You are talking about Multitenancy, or multiple separate entities sharing code, but having its own dataset. While there are many ways to approach this, the most simple way to approach this is by creating multiple databases (schemas), where each project only has access to its own, separate database.

If the amount of data that each user creates is very small, it can make sense to use a single table or set of tables, where each one is prefixed by a user id, or user field, so the applications controls you can only write and read your own rows. That assumes that you ha full control the application.

If the application is on the user's side, you shouldn't solve this on MySQL only- you need a backend application communicating to the user's frontend application, and being between the database and the frontend application for access control (implementing authorization and authentication).