It isn't all that complex. The desired bacteria need 4 things: 1. A surface to which to adhere. 2. Oxygen. 3 Food (ammonia and nitrite). 4. Water.
Bacterial colonies are constantly adjusting in two ways. First, they size to the available sources of food and oxygen- add more food and they multiply, decrease the available food and they die back. Bacterial colonies will always size up or down in this respect. Second, the individual bacteria are always dying off in some places and being replaced with new ones, so the concentrations in a tank while they may be stable in terms of numbers are not constant in terms of where the individudals may be. A corallary of this is that the colony will tend to outgrow the food source and then die back to the appropriate size.
The bacteria are opportunistic and colonize the greatest where the best/most available sources of food and oxygen are (bio-wheels are designed to create an oxygen rich home). Normally this is inside filter media. However, any hard surface in a tank will act as a home to the bacteria. The top part of the substrate is another good place for bacteria to colonize due to the water bringing food and oxygen to a vast surface area where bacteria can live. As you move deeper into the substrate the circulation drops off sharply and the substrate becomes much less habitable. This is the principle behind ugf/rugf- by forcing water through the substrate it allows a large volume of "media" to get plenty of food and oxygen carried in by the water moving through the gravel.
The purpose of seeding a tank is to jump start the cycle. The more bacteria you start with, the faster the colony can reproduce to consume the available food sources. However, the other side of this coin is when you remove bacteria from tank A to seed tank B, you must be careful not to remove too much from a single tank or the existing tank will have a bacteria shortage and you get an ammonia spike there.
Compared to other bacteria, the good ones in a tank are relatively slow growing. Under the most optimal conditions it will take 12 hours or more to double in size. That is why seeding can greatly accellerate the cycling process- the more you start with, the faster they can "double".
One misconception about filtration is that more = more bacteria. If you have a fully cycled tank with a single filter on it and you add a second identical filter, you dont get twice the bacteria. What you get in the longer run is two filters each with 1/2 the bacteria the single one had. Filters don't feed bateria, they just provide optimal homes for the number a given tank needs.
I hope this has cleared up some of the mystery
