VPN stands for “Virtual Private Network.” When you connect to a VPN, all data that you send (such as the requests to servers when browsing the web) appears to originate from the VPN itself, rather than your internet service provider (ISP). This masks your IP address, which can be an...
You can combine VPN and DNS-based ad blocking, usually. Mullvad has it’s own dns server with ad filter. However you can use any other.
I don’t think Android supports two different VPNs.