Can my iPhone be remotely monitored or hacked

There is no know method to remotely hack an iPhone no.

An Apple ID on the other hand is a completely different thing. It can be hacked or accessed without permission by different mechanisms. To minimize the chance of this happening it is suggested you use a strong password and enable Two Factor authentication.

Link-> Two-factor authentication for Apple ID - Apple Support

Your phone won't be hacked or get viruses unless it has been jailbroken. It can get spam and malware if you are not careful which sites you visit. Check this link...

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support