When using a VPN to download things or visit websites your ISP cannot see or know what you're downloading.
The ISP cannot even see what websites you're visiting or see what you're downloading when you're using a VPN.
However the ISP still knows who you are and your regular IP address even when you change your IP address through a VPN.
Using a VPN helps prevent you from getting into trouble when doing things such as downloading movies, torrent, etc.
But if your ISP detects an unusually large amount of traffic and downloads coming from your internet account then they can still investigate and then terminate or suspend your service.
So you have a less of a chance at getting into trouble when using a VPN and it does help protect you as well from being hacked but your ISP still knows your IP address that they assigned you and they know your physical address as well.