Using EQ and stereo positioning, one carves out "space" for the vocals, making sure that no instrument is playing in the same frequency range and stereo image position. Judicious use of reverb allows you to also move an audio source forward or backward in the image.
Well, using an excellent vocal chain (great mic, pre, comp EQ etc....) helps a lot when you need to record a vocal that still sounds natural, but lives in the top 15% of its dynamic range. Good comps on the guitars can also help them seem bigger and yet actually be lower in the mix. Couple that with good mastering, and suddenly you have a loud full mix with huge instrumentation but a vocal that still slices its way to the top. This is exactly why people still use the big boys