I recently tried to figure out where DALYs come from.
After a bit of searching, the best I could find was this report on the origin of the metric (the first Global Burden of Disease assessment). The report includes this explanation:

And:

But I'm left with many other questions:
- How many health workers were consulted?
- Were people other than health workers consulted, especially people who have themselves experienced the relevant health issues?
- Were DALY values updated in successive instances of the GBD?
- Are transcripts of any of these "formal exercises" available somewhere?
Ideally, I'd love to find a document/video that covers DALYs in the style of a factory tour video; I want to know what goes into them, who is involved, and what the creation process looks like.
Does anyone know of such a resource, and/or the answers to any of my questions?
I just had the exact same question, so thanks Aaron for asking this, and Derek for giving this answer :)