Why buttermilk matters in a recipe beyond just "milk"
Buttermilk's acidity does real chemical work in baking, not just flavor — it reacts with baking soda to help batters rise, and it tenderizes gluten, which is part of why buttermilk biscuits and pancakes turn out so much more tender than ones made with plain milk. That's why a simple milk-plus-acid substitute works well: the acid (lemon juice or vinegar) recreates the tang and, more importantly, the pH buttermilk provides, even without buttermilk's exact thickness or complex flavor.
Which method to use when
Milk plus lemon juice or vinegar is the fastest, most universally available option — both work almost identically, though vinegar is slightly more neutral in flavor if you're concerned about a lemon taste coming through. Yogurt thinned with milk gets closer to buttermilk's actual thickness and tang, useful if you want a result closer to the real thing rather than just the chemistry. Cream of tartar is a good option if you're out of both lemon and vinegar but happen to have it in your spice cabinet from meringue-making.
FAQ
Does it matter which milk fat percentage I use? Whole milk gets closest to real buttermilk's richness, but low-fat milk works fine for the chemical reaction — the fat content mostly affects final texture/richness, not whether the substitute "works."
How long should I let the milk and acid sit? About 5-10 minutes at room temperature — you'll see it slightly thicken and curdle, which is the sign it's ready to use.