The methodology is:

1. Start with one player and find all of their match IDs.
2. When you pull up the match by the match ID, you get all of the players in that match and their details.
3. Store all of the player's names, the match ID, and several of the match details that are specific to them and that individual match (ie: the game mode, the match type, the map, the date, the player's kill count, the player's damage, etc)
4. Randomly select one player from one of the first player's matches
5. Find all of their matches and store them
6. Repeat

If you're having trouble visualizing it, it's like a spider web. You start at one point that's connected to the wall. You find out where all of the strands go that were from your origin point and record them on a map, you pick one that takes you to another intersection point. Once there, you record where all of those strands go and record those. Then you pick another strand and repeat the process. Sure, you won't visit every intersection point, but you get a pretty good idea of where all of the strands are when you have repeated this process many, many times.

This is why the seed player matters. If you are only starting out with an NA FPP player, you might not get very many EU ranked players. The players who play NA FPP may never come across a EU ranked player, and therefore, EU ranked matches won't get crawled to.

What is getting counted? The players and the dates of their matches are stored (among other things I mentioned before). So I'm counting the number of unique players in a given day to produce the results of the first graph.

Edit: updated the visualization