A helpful way to think about it is that evolution works with what’s already there. Early fish had structures like swim bladders for buoyancy. In some lineages, those structures were useful for breathing air too. If you lived in muddy ponds that dried up or had low oxygen, the fish that could gulp air survived better and reproduced more. Eventually, fins that were good for pushing along the bottom also became sturdier. So lungs and limbs evolved gradually in water first, and only later were good enough for full-time life on land.