It used to be muffins were made using the "muffin method"
1. Sift together the dry ingredients.
2. Combine all liquid ingredients, including melted fat or oil.
3. Add the liquids to the dry ingredients and mix just until all flour is moistened. The batter will look lumpy. Do not overmix.
4. Place in muffin pans and bake immediately. The dry and liquid mixtures may be prepared in advance. But once the mixtures are combined, the batter should be baked without delay, or loss of volume may result.
Cupcakes, conversely, could be mixed using any cake mixing method from the two-step method, melt&mix, all-in, sponge-cake, creaming method to any combination of method.
As with most cases, the demarcation point is blurred and what-ever works, goes!