I have done brisket with out having to add fuel. Infact didn't start with a full basket. I still had leftover fuel at the end
Air is coming in all around the basket. If you will look at my basket it has legs. I just dump lit charcoal on top and let it burn down Their is about 9000btu's in 1 pound of charcoal. The UDS is easy to control air on. So adjust your air to the temp you want and go to bed.
Like mule said...air comes in from all directions. My basket doesn't have legs, the basket sits in some U-Bolts. I have 2 sets of U Bolts, A lower set for smoking and a upper set for grilling and searing.
When i want to sear i have a hook and lift the basket to the upper bolts.
Burn pattern.... everybody puts their lit coals in different spots i think... I like to just light the edge of the basket and let it burn out. It makes it nice because when i shut down i always have fresh charcoal left and the partially burnt coals is the starter for the next cook.