Fitness

Back extensions are king. Try to work up to 30 reps, then add a little weight. The low back pump is unreal

Squats and deadlifts activate the lower back in a stabilizing fashion, which is almost always the way the lower back is used.

Hyperextensions can allow you to target them directly as assistance work, however, for example.

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.

Squats are primarily a leg/quad exercise but they are irritate my back injury so I’m glad I’m able to get back to them. Squats definitely require back strength even if that isn’t the primary purpose. The classic all around posterior chain/lower back exercise is the deadlift but hyperextensions/good mornings and other variations all are good too.

What do you mean by "building up lower back muscles"? Understanding your goals would be helpful. Honestly deadlifts are a one stop shop and by far the best IMO, but I also like one leg stuff because it requires different stabilization (e.g. lunges, step ups). Planks help balance out the core too. The overwhelming majority of people would benefit from regular deadlifts, and other than specific sport needs or mobility issues, are enough.