This is a fundamental premise of Mayayana, that the goal is not to be extinguished and leave samsara like an Arhat, but to stick around until everyone is enlightened.
Here is the zen formulation of the Bodhisattva Vow (couldn't find the Tibetan version)
Beings are numberless; I vow to save them. Delusions are
inexhaustible; I vow to end them. Dharma gates are boundless; I vow to
enter them. Buddha's Way is unsurpassable; I vow to become it.
As for where one is upon enlightenment, there are more lots of formulations, I don't have a good idea on how to summarize them.