You can notify course participants in different ways.

In the Forum “Announcements” or in a general forum everyone in the course room can read along. Personal messages ✉️ (messages) can be sent to single or multiple people. To be distinguished from this are notifications 🔔 (notifications) that are triggered by events, e.g. by new posts in the forum.

This is how you can send a ✉️ message to course participants:

1. in your course, go to Participants in the navigation.
2. select the relevant participants: “Select all” with the button below or Select individual by placing check marks in the “Select “ column.
3. at the bottom of the window: “For selected users…” select “Send message” in the drop-down menu.
4. enter your message and confirm with “Send message to x people”.

See also https://docs.moodle.org/en/Notifications

Notifications 🔔 are triggered automatically and inform teachers, students and other users about events in Moodle, such as new forum posts or assignment submissions that need to be graded.
New notifications are indicated by a number on the bell at the top right of the user menu. You can set how you want to be notified about new notifications.

more on notifications moodle.org

“Mail” is an extension to Moodle. With “Mail” course related messages can be sent to participants in Moodle.
The extension “Mail” is offered as an alternative to Quickmail (was active until 7.9.20). You can access it from the menu bar next to the profile settings by clicking on the “envelope” icon.