Heaven is the mind of God, and to travel through it is to travel through both Its brain and Its thoughts, the dreamer and the dream (which makes me think of a setting where Cthulhu is God, or at least His Incarnation). Heaven is, as it were, a sort of computer. Heaven operates outside of space and time as we understand these things and may set aside processing power to simulate chosen lives based on the total data that is available to it and, if so desired, make changes so that it is better-suited to its intended task (if there is any at all). These are the Streets, and they may come into their position as a reward for their services or because their experiences are necessary for a task at hand. At a whim their memories and personalities may be rewritten, which leads some to wonder on such things as authenticity, and many eventually choose to consider themselves separate entities from the mortals they simulate. Because Heaven is beyond our conception of the universe the Streets may be placed in times before their mortal births. Streets may also go offline at times, which they experience simply as an instant transition to the time and place that they are next brought online.
"Angel" is the most common term for God's servants. They are themselves God, or fragments of Heaven, by being a part of Heaven that intrudes into our universe (in one sense they can also be considered Christ). Angels may be Streets or rawer portions of Heaven's substance, and are not quite Heaven itself: If a Street is sent to the universe what happens s that a copy is made here and then, at the conclusion of its assignment it is destroyed/unwritten/deleted and its original counterpart in Heaven is rewritten to include all the experiences of the copy.
The closest that anyone can figure out about God's motives is that It is "drawing the universe toward Itself" as if it were some kind of Omega Point in order to transform our universe into a counterpart and true equal, or else integrate itself into and in some sense assimilate our universe. Regardless, there are a number of situations in which its angels have bred with non-Heavenly lifeforms, which behavior can be explained by either hypothesis.