Do Avast Antivirus act as Anti-malware by default, or we should enable it in settings.
Yes, all AVs are also protecting against malware. But like most things in life nothing is 100%
Viruses are much less common, now the most common issue is malware, which really includes all types of malicious (the mal in malware) attacks.
Whilst there are some specific anti-malware programs most antivirus programs also include malware detection.