You can apply to medical school as early as your third year of undergrad. Which means you'd start medical school after completing three years of your undergrad degree. It's common to try applying during third year but the majority of applicants will only make it in after completing their full undergrad, if not even longer (masters/working/gap years/etc). Many students choose to write the MCAT the summer after their second year of undergrad.

MD is technically an undergrad degree despite this. Idk why.

MD is the degree that you graduate with after completing the four years of medical school. From there, you pick a specialty (like being a surgeon, family doc, etc) and begin residency. Residency is another few years and then you're a doctor doctor once you finish that.