Bear with me! I have never used vpn yet

If I get an vpn account ie. PIA, purevpn or whatever. I can add an PPTP or OpenVPN in the nas? But if I buy an account then what? Can I use all providers or?
My concern is when I use download center or an bittorrent software. If I get a vpn provider will all my trafic when I download pass through the vpn provider to my nas or do I have to do something else?
Anything else?
Thx
