Browse:
Type a message in a reply or start a new conversation.
Type @ and the person's name.
Select the person's name you want to mention.
Finish your message and select Send.
Type @team or @channel.
Select the team or channel you want to mention.
https://support.microsoft.com/en-us/office/get-attention-with-mentions-b2ffb135-7069-4880-84ee-5b27f402418b