As noted - the Monster Manual does have an Index - at the very back. Lists all the monsters, by the page of their stat blocks.
As for the lore, never cared for it much - since I homebrew and come up with my own lore how creatures are made. Sometimes, I use the basic information - and develop it further.
As for the number of appearing, I'd chalk that up to your party size vs the DC of the monster(s).
And you can chalk up that most humanoids (goblins, orcs, kobolds, hobgoblins, kuo-toa, etc., probably all have villages somewhere - but it's probably unlikely the party will march into a village of orcs, unless there's some kind of alliance).