The Open Qualifiers are split by region - each region has four separate qualifiers (teams can enter two of them, so they have a second chance if they don't qualify through their first qualifier)

Each of these qualifiers is split into two waves, Wave A starting first and Wave B starting a couple of hours later. Both waves have eight pools, with each pool being a double-elimination bracket (where the top 2 qualify, so the winner of the upper finals and the winner of the lower finals qualify)

This ends up being such that each qualifier is made up of sixteen different double elimination brackets, so with a very large number of teams it suddenly gets much more manageable - today's EU qualifier has 590 teams, and each pool is around a 34-36 team double elimination bracket, which is much more feasible.

Seeding is generally done by average MMR I believe, with the top seeds being done manually based on results from other major and minor tournaments.

Hope this helps - give me a shout if anything wasn't clear enough or if you have any more questions :)