Always switch to the native app. If a pop up claims your iCloud is full, close the browser and open the Settings app. If the warning is real, it will appear there.
Phishing pop-ups are designed to look legitimate, but they often have distinct "tells": phishing pop ups
Using loud noises or full-screen modes that are difficult to close to pressure you into acting quickly [5.12, 5.13]. Tech Support Scams: Always switch to the native app
Legitimate companies rarely have typos in their official alerts. phishing pop ups