You can refer to https://learn.microsoft.com/en-us/graph/api/chatmessage-post?view=graph-rest-1.0&tabs=http#example-2--a-user-in-a-channel-message on how to send message with an atmention on it. Sending a message with atmention automatically sends a notification to the target user.
If you want to send notifications to user in Teams (for something other than at mention for example) please refer to https://learn.microsoft.com/en-us/graph/teams-send-activityfeednotifications