The concern with lead left behind is left behind in the air for you to breathe.
The topic of bullets could be a doctoral thesis in itself. The simplest is cast lead, but even that gets into bullet shapes, lubes, what lead alloy is used, hardening of lead, etc etc.
Once you get into partially jacketed, full jacket, partition bullets, etc, it becomes a large book.
What specifically were you looking for?
What we have heah is.... failure to communicate.