It’s mostly water vapour not water droplets. When the vapour coaleses around something (bit of dust etc) it forms a water droplet which is more dense than the air and therefore falls .

I’m not sure on the exact science but there are also significant updrafts in big cumulonimbus clouds. When the water vapour reaches a critical height it freezes and starts to fall. Friction between these falling ice crystals and the other water vapour can charge up the cloud and eventually cause lightning. Updrafts in the clouds can slow down these falling crystals and cause a recirculation within the cloud allowing huge charges to build up.