The logic is quite simple: When not commented out (through #) the first mirror found and checked is used. While downloading data a problem in resolving would occur the next mirror in the line is used. Annotation: For sure only related to the corresponding used as for "stable" or "testing" (or "hidden" when using tor).
If you like to create a mirror you can use different tools like lftp.
Human being in favor with clear principles and so also for freedom in soft- and hardware!
Certainly anyone who has the power to make you believe absurdities has the power to make you commit injustices: For a life of every being full with peace and kindness, including diversity and freedom. Capitalism is destroying our minds, the planet itself and the universe in the end!