Sometimes, it depends what is contaminating the water. Boiling kills many common bacteria like salmonella and e-coli so water centers will send out boil advisories if they detect those types of contaminations and you will be fine in that case. Sometimes it isn't bacteria themselves that make you sick, but the waste products and those will remain after boiling (especially common if the water was stagnant). Sometimes water is unsafe because of chemical contamination and that is also unaffected by boiling.

Some bacteria and viruses can survive boiling, so it is more of a reduction of risk rather than truly making it safe.

Depends on where the water came from. Many things like heavy metals and harmful chemicals remain.

It would be a good idea to look up the information for yourself since you shouldn't take advice on Reddit. But mostly to know more about the different situations with the water you may come across.

To add to what others are saying, if the problem is chemical contamination you fix it by distillation, aka more boiling.