i watched a video on pray pixel about a street photographer with a Leica film camera. So I read a bit about the mechanical Leica, and it said there was no built in light meter. Hailing from the DSLR era, how would one have calculated their exposure triangle without the advent of a built in, or hand held light meter?
What you're looking for is the Sunny 16 rule.
The wikipedia page is good enough; however, all of the SE sites don't like you to dump a link and leave. So, I'll just rip the rule straight from the page.
Since you know the exposure triangle, it boils down to: at ISO 100 you use f/16 and 1/100s on a sunny day to get a properly exposed picture.
With that as a basis, you can adjust yourself through stops to get the same exposure at ISO or at f/32.
Although there was a time prior to hand held light meters, they are pretty darn old...mine is from the 1940's and still works just fine. In face it is better than the built in one on my AE-1.
Here is the photographic formula for the calculation of exposure used for photographic emulsions. Electronic ISO speed designations used today is an approximation of the earlier ASA speeds derived in the early 1930s. For all intents and purposes ASA and ISO are interchangeable.
The f/# = sq. root of ASA
shutter speed = 1/(candles per sq.ft.)
Such exposure would produce a patch with a density (with standard sensitometrical processing) of 0.301 over base plus fog density. Plus or minus : ).