Helping Others With Their Macs Using Screen Sharing Through Messages

 https://www.youtube.com/watch?v=Vc-shCDeqH0

Connect to Another Mac's Desktop Via Screen Sharing

Screen sharing capability is built into the Mac. With it, you can access a remote Mac's desktop, and view and manipulate files, folders, and applications, just as if you were sitting in front of the remote Mac.....

https://www.lifewire.com/mac-screen-sharing-connect-to-another-macs-desktop-2260828