If I log into someone else iCloud would they know from their phone?

If they have 2 factor authentication enabled, you can't sign in unless they approve you from their device and if they don't have it enabled or your device has already been authorised, they will receive an email that you have signed in.

YES, notwithstanding 2 factor identification bc it will be noted in the device history. Do not log into someone else's account please. The security and escalations departments are overwhelmed as is and take days to return phone calls to the victims of breaches, etc.