Question about marathon races

I haven't really been interested in races before,
so I've just been running along the riverbank for about 3 years.
But I've recently developed an interest and am thinking of entering a half marathon or a 10km race.
My questions are:
1. For non-elite runners, how are bib numbers assigned?
Is it first-come, first-served based on registration,
or is it random?
2. In crowded races, if I get a high number and end up running much further back,
is there any disadvantage in terms of timing compared to the people in front?
Or do they account for that
even if I'm behind?
Since I've never participated in one before, I'm asking these two basic questions.