Fitness

Back extensions are probably the best ones as those directly train lower back muscles with full range of motion, but any deadlift variation, good morning and rows without chest support do train lower back in isometric holds as well.
You're not wrong, squats are mainly for legs, but as those require bracing it trains core muscles as well and lower back falls under core muscles too, though it's questionable how effectively it trains lower back muscles in comparison to direct work.