Questions about Marvel Realities

So, I recently found myself confused about the Marvel multiverse and how they work. For instance, Dr, Strange is the Sorcerer Supreme of Earth 616. Does that mean he is the Sorcerer Supreme of that entire reality or just Earth? Are there other Sorcerer Supremes on other planets or should Strange be fighting mystical threats all over the Universe? And are other dimensions(Asgard, various heaven, and hell realms, limbo, etc) part of the Earth 616(or whatever reality they are attached to) or are they considered different realities all together? And how do the Incursions work? Do the collision of two alternate reality Earth's destroy just Earth or the entirety of the reality? Basically, how does the Multiverse work?

The way I understand it is that different dimensions like Asgard, etc. are all part of the same Universe. Its only when you get into alternate versions of the same area that you're dealing with a different universe.

For example the main marvel reality is the 616 universe, the 1610 Universe is the Ultimate Universe, the 311 universe is where the series "1602" was.

An easier way to think of it, is- every time you make a decision, potentially a new universe is created. Universe 1 is where I went to work today, Universe 2 is where I slept in and stayed home.

As for Incursions I haven't read those series, but I'd guess that Earth was the focal point, but the two realities/universes would be destroyed.

@cave_duck: this is pretty much it