How does the Yandex Direct time planner work?

Question

MySQL

How are tasks stored in the database and how are they executed?
After all, there are many users and many tasks are obtained.
Maybe task runs every minute and looks at what tasks need to be completed now?
But then it may turn out that one user has more tasks and while they are completed, others will not have time. You can of course use round robbin by user id.
But it's still not clear how best to store it in the database.
I wonder how best to store pending tasks and execute them,