Time, money (from rich kings that paid them to do art), and access to knowledge from each other (ex: Many Renaissance artists knew perspective as a result of their affiliation with each other). Also, keep in mind, a lot of "Renaissance" artists were really crazy smart people that knew math, science, etc, and could do circles around the average citizen today.
Ex: If you've never seen DaVinci's helicopter prototype drawing, you'll be fucking amazed to know that he thought of it and tried to conceptualize a fucking flying machine before the Wright brothers ever took their plane to flight in North Carolina.
What you can do is basically trace their works, and determine basic shapes. What does this allow you to do? It teaches you composition from the fucking masters themselves. You'll start to see patterns of how figures are placed, especially in the perspective of a scene, and "Why" a particular figure stands out in a scene versus others are just background characters.
What's cooler is that you can take it one step further and then interlace your own characters in the same poses, and have different lighting effects as well as different color tones to totally change the mood of a piece. Its fun to copy, Anon, you can only learn from it.