Coolant and distilled/demineralized water posses unique densities, or specific gravities, like your fave ale or dry Irish stout (yet don't be drinking no antifreeze or coolant mix, k?).

Thermal-induced currents, the water pump transfer volumes, and mostly so the solubility between antifreeze and distilled/demineralized water facilitates a coolant mix after sometime in the water jacket. (Don't ask me for proof rigging up my camera or testing equipment to a hot B30 E engine.)

That stated, I pour both antifreeze and distilled/demineralized water into the coolant expansion tank simultaneously; yet the antifreeze container empties before the water empties from the distilled/demineralized water containing container so the outcome is the correct mix.

Remember folks, do not ever use tapwater when mixing with antifreeze to make coolant.

Hope that helps.

Buttermilk and Corn Chips for Dinner Guy.
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Dood, you said, "buttermilk."