Let's first say that if you were told that malware is not an issue on iPhones, then you were misinformed. Bad guys are out there and they target EVERY platform.
With the Note8, best practices would be the same for the iPhones. Don't root/jailbreak, only install apps from trusted sources like the play store or Amazon's app store. Stay away from open public networks, keep your data encrypted, enable two-step authentication.
The truth is that there are not true viruses (self-replicating at the hardware layer) anymore that propagate in the wild. The malware you are most likely to encounter is some form of scripting that execute existing commands to steal data, or compromise your account. That happens more through social engineering than by brute force or programmatically. The exception, of course, is if you install hacked apps which have malicious code built into them.
How do you know you've been compromised? Unfortunately, sometime you won't ... not until it's too late. That's why it's important to protect your information. I'm not a big fan of security apps. Most of it is smoke and mirrors IMHO. Follow safe practices and it becomes unnecessary.