This is probably a better question for Sound.SE, but anyway:
A good vocalist will be aware of their mic placement (it becomes habit pretty quick). In "loud" genres, rock for example, as an engineer, you typically want the mic very close the vocalist's mouth. The closer the vocalist (and louder the vocalist), the less gain you require, which improves your gain-before-feedback, which is typically very important in loud live concerts. So, in short, most vocalists are pretty good about this (and it's a nightmare to work with someone, vocalist or presenter, who doesn't know how to hold a mic). Also, the way humans project sound when singing loudly or screaming is very different than speaking. You'll find that when you're speaking into a mic or singing into a mic, the directional behaviour of your voice is drastically different.
Do they have some kind of automatic electronic system that keeps the levels balanced?
Compression is an audio process which reduces gain above a threshold, and sort of does this "level balancing" you speak of. However, heavy compression on live concert music is suicide for a sound engineer, as it drastically reduces gain-before-feedback, which is bad. While I might use a tiny hint of compression (or more likely, limiting) on a vocalist, it wouldn't be applied to such a degree where it would sound like a "level balancing" effect.
There are some special cases to consider:
If you are watching a video of a well run concert, and everyone on stage has in-ear-monitors (IEMs), the "stage volume" (how loud the stage area is) can be reasonably low. The audience has huge directional line arrays blasting their direction, but hopefully the stage volume is low because everyone has IEMs. In this case, the gain before feedback margin is higher, and a sound engineer has a lot more freedom to compress vocals, etc. In this case, the sound in the video might be a board mix, which is directly from the mixing console, not a camera microphone in the crowd, which gives a further boost in audio quality and (if well mixed) can let the vocals "shine" far more as they aren't overpowered by instrumentation.
Also, if you are watching a video and it sounds absolutely wonderful, perhaps it's not actually the sound from the concert. This is common in music videos and such.
Sorry for the wall of text, but I don't think there's a concrete answer. Hopefully this helps.