What does draft (扎啤) beer actually refer to?

Answer: Beer served from a keg through a tap

Beer served from a keg through a tap ✓ — Correct! 'Draft' is a serving method, not a recipe — the beer is pushed from a keg through a tap instead of poured from a bottle or can. Your favorite lager can be sold both ways. Draft often tastes fresher because it's kept cold, dark, and turned over quickly.

A special stronger recipe — Wrong. Draft isn't a stronger or special recipe. The exact same beer can be sold in a bottle and on tap.

Beer brewed without hops — Wrong. Draft has nothing to do with hops. It only describes how the beer is dispensed: from a keg via a tap.