How to properly use g-code converted (CAM software)

Hello there,
I am a member of the CAD/CAM group and I was following a trail
regarding what to use as a G-code
interpreter.
I have a question I wanted to ask directly. I am a CAD junkie from
way back - I use autoCAD, Pro E,
Unigraphics, etc... Likewise, I have a manual mill and manual lathe
and do quite well - but I am now
in the CNC arena... Anyway, I do not understand how this G-code
conversion works... So let me make
a statement & question and please tell me if I a correct or WAY off.

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When I draw my designs in AutoCAD, I have the traditional front, top,
side view (& sometimes ortho)...
Milling is done from the top down. So for a g-code converter, do I
simply import a top view ONLY -
then have it converted?
If so, how does it know how deep to go, or what if I have a round
profile that cannot be seen from the
top? This detail is missing no matter what website I go to and I
have not been able to figure it out.

Likewise what is the proper method to convert (entire drawing,
etc)... ? Any information on this would
really be helpful. Because I have details drawings that produce some
crazy stuff when I import them...