Regional Cooking

I cook general 'Mexican food' and have been for two years. I want to be more specific and regional but I don't know what the pantry differences are. If I wanted to cook Oaxacan vs Yucatecan vs northern Mexican food at home, what different ingredients would I need?

I keep seeing epazote listed in bean recipes and I cannot find it locally. Some recipes say it is essential and others do not mention it at all. What does it actually contribute and is there a substitute?

I see this instruction in every dried chile recipe — toast briefly before soaking. But I skipped it once and my salsa tasted fine. Is this step actually essential or is it one of those instructions that got passed down without anyone checking if it matters?

I know the burrito has a complicated regional history. Beyond the obvious Tex-Mex vs Mexican distinction, are there regional variations within Mexico or within the US that use distinctly different approaches to the same format?

I keep seeing these three chiles in recipes and they are described as interchangeable in some sources and completely different in others. I cannot figure out which is correct or when the differences matter.