How do I see if someone is on my iCloud via web

Apple ID device list -Check to see where you‘re signed in

You get mail messages about “new” logins. Devices that have not logged into iCloud before, or sometimes that have been recently upgraded. Disassociate any devices you do not recognize within your Apple ID and do not still have, and change your Apple ID password. These “new login” mail messages sometimes also arrive after an update or change to the Mac.

MacOS High Sierra is thr macOS version 10.13; the version prior to Mojave 10.14.

Start here: https://appleid.apple.com