If upstairs bathroom leaking water downstairs, how how to water-proof the bathroom ? What do the worker do to water-proof the bathroom ?

Leaking on the floor or near wall?

If not major, it can be just DIY or simple job. Nowadays you can buy liquid to be poured on the flooring and let it flow in cracks and plugged the leakage.

The original floor, the water proofing substance is mixed with the concrete during construction. Simple one in olden days would be painting the floor with water proofing solution before a layer of cement and floor tiles. This method is used on the wall as well to certain height (maybe about a foot).

The leak cause a hole downstairs, and whenever use the bath, water will drip from the hole. Is this minor or major ?

This is minor. Quite common the drip is from the water drainage hole. I believe the water seeps between the PVC outer edge and the floor concrete. Probably you can just pour the sealant and see if this work. It should work.

I would try something like this and pour around the drainage hole. I believe there is exposed concrete not covered by tile around it as well. Pour and let it dry.

https://www.lelong.com.my/pye-krek-seal-res...7-01-Sale-I.htm