Hey Mitchell
Good questions.
AWS has a number of codes with prequalified joint details. D1.3 (structural sheet steel) and D1.6 (stainless structural steel) are other examples.
The prequalified base metals in D1.1 and the base metal/filler metal combinations that are prequalified are steels that have been around a very long time, and it is know how they will perform under certain conditions. Those certain conditions are the "essential variables" listed in clause 3. Joint configurations are also part of those certain conditons... It is known by years of experience that the prequalified bevel angles, root openings, root faces etc found in the prequalified joint details will work if the other essential variables are followed.
As far as "hundreds of types of metals" Remember that D1.1 prequalification is limited to the base metals found in Table 3.1... And many of those are very similar, possibly only seperated by Grade.
The Canadian Welding Bureau (CWB) has prequalified joint details in their structural steel welding specification CSA W59 It is similar in some aspects to AWS D1.1 with some notable differences... Such as a unique set of joint details for both GMAW and GMAWP.