Small Engines (Lawn Mowers, etc.): Diagnose a leaky carburetor chamber and learn the intended function of key parts, rubber gasket, commercial walk behind mower

Question
I have a John Deere GS45 commercial walk behind mower with a Kowasaki engine. When I started the engine today, I noted a steady flow of fuel drips coming from what I believe is the carburetor chamber (bowl?). The chamber has a bolt w/washer that is screwed up into the center of the bottom of the chamber. I removed and inspected and replaced the bolt and washer (looks like a snall rubber gasket). The leak persisted. Should I assume the obvious, and order a replacement gasket (if I can locate the p/n and find it in inventory somewhere). Please advise. The other part of my question, is to ask you to explain why the chamber has a bolt with a gasket. Why not simply a uniform, leak proof bottom to the chamber? Note, this chamber also has a screw w/compression spring on the side. Does that control the air mixture with the fuel? Carburetors (and small engines) are new to me. My main goal is to stop the fuel from leaking.  An added bonus would be to learn the intended function and operation of a carburetor so that I can keep this mower running for years.

Answer
The bolt on the bottom is there so you can disassemble the carb and clean it, which you need to do to solve your problem.
The inside of the float bowl (this is what we call the bottom chamber) actually works just like your toilet.
Inside the chamber is a float and a needle valve. As the fuel fills the bowl the fuel level raises the float pushing the needle valve up, closing the fuel inlet valve.
Your toilet makes an annoying noise when it's float sticks and you go jiggle the handle. Unfortunately your engine has no handle to jiggle.
Remove the carb and clean the needle area to stop the leak. It's really pretty easy.