Strange battery use, pop-ups, unfamiliar apps, or unusual account activity can have many explanations. Do not assume every unusual phone behaviour means the device was hacked.
Start by securing important accounts, preferably from a trusted device if you believe the phone itself may be compromised. Change important passwords and review recent sign-ins and connected devices.
Check installed applications, device permissions, accessibility settings, and other security-sensitive settings for changes you did not make.
Install available operating-system and security updates. Remove applications you do not recognize and use the device's built-in security protections.
If you find evidence that an account was accessed, use the relevant account-compromise guidance as well as the device-security steps.