In article , Tom E. wrote:

Where did the bacteria come from???

It is in the air.

And why is ammonia added in the process?

The "Cycle" is the process where bacteria convert ammonia to nitrite,
and then to nitrate. Tanks with a plenum or deep sand bed go furthure
and convert nitrate to nitrogen gas. The bacteria need a source of
ammonia to eat. Pure ammonia, decaying shrimp, live rock die off, or a
live fish are all sources of ammonia.

Fishless cycling is the smart way to cycle a tank.