Testing conundrum. Who knows the science?

Ok so while I sit here waiting for my tank to cycle I've been reading and thinking a bunch. If I'm not mistaken, transferring "established" tank water does not transfer the beneficial bacteria only live rock and live sand.



That being said, if the bacteria doesn't live in the water column, hoe and why do we test the water column for said bacteria?? Wouldn't we need to scrape the rock?



I'd like to know the science behind this one!