Develop a notification module and send to email in CRM on YII2

There is a CRM-system for setting goals and objectives for employees of the company, developed on YII2.
Inside the system there are notifications that are displayed on the main page of the user.

Type of notifications in the system:
1. Setting a new task.
2. Notifications to the crown, about the approaching deadline for the completion of the task and about the missed deadline
3. Comments to the task
4. Draft solution to the problem and as a message
5. Messages inside the CRM-system

It is necessary to understand someone elses code, develop a module, and configure the sending of notifications to the users email.

For all events in the CRM-system, we send an email message "New event in the CRM system, your participation is required."