Remote Access, Bluetooth used in hacking pc?

I do not know much about remote accessing other than when I have used it for support from my pc manufacturer. How would someone other than that gain control of the ability to access a pc? I have found some interesting events concerning this and was wondering. Also, if someone had the router settings for a wi-fi device, how easy is it to gain access to that network, account and control everything? Is this how "monitoring" programs operate?