How do I secure my iPhone and iCloud?

You check the devices logged into your account by going to Settings > [name on top], and scroll to the bottom. That will tell you the devices logged into your Apple account.

Where are you receiving this Notification:

• If you got an email/message, then consider that many times scammers will send you those so you call a number or click a link and enter your account credentials. In that case, you have given the scammers your Password and they can now compromise your account.
• If you see this on a service such as Google or Facebook, then you should contact them. Many times the devices they show that are logged in are not correct as your device is not broadcasting that information and it is just a guess by that service based on "fingerprinting". It could also be that the Google or Facebook account has been compromised and it has nothing to do with your Apple Account or your device.

Saegzz wrote:

What does it mean if they are logged into your account? I checked mine and 2 tvs were listed. I’m guessing from when I used airplay. Does this mean they can access my data?

If you used those TV's to log into Apple TV+, then they will appear there and you can either sign out on the TV or tap in in Settings on your phone and it will be removed. There is no access to your password on that TV, but a person could continue to use the service there as if it were you.

I have not seen a TV added when I AirPlay content to it.

What does it mean if they are logged into your account? I checked mine and 2 tvs were listed. I’m guessing from when I used airplay. Does this mean they can access my data?

That makes sense. I used to have Apple TV