Should I buy antivirus software for my phone and what is best

None. You would just be wasting your money. Your phone is not going to get a virus and there is no application that can scan your phone. The apps in iOS are sandboxed so they cannot access other apps, so anything that claims it can scan your phone is straight up lying to you.

None. Apple's built-in defences stop malware from affecting it and stop any scanners from checking files other than their own documents.

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None. You don't need it. Most are a scam anyways.