How do free full version antivirus providers work and survive without money?

Avast, avira are free antivirus and after installing them, no need of worry as they are infact 100% effective. No need to purchase antivirus. How do they survive without money? How can they give best for free? Norton, Mcfee, Kaspersky etc. paid antivirus are **** when compared to these two free antivirus.

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).