As I understand, here are the Mahayana and Theravada conceptions of nirvana/nibbana:
As do several later schools of Hinduism, notably Advaita Vedanta, with which Mahayana/Madhyamika philosophy may have merged to some degree over the millenia, Mahayana thinks that the relativistic, dualistic (self vs. other) reality in which we live is not real. Though both Mahayana and Theravada would probably agree that our current reality is empty of a permanent/stable essence, because everything in this world somehow depends on something else (e.g., consciousness depends on the body, the body depends on molecules and atoms, etc.), Mahayana claims that our current reality is not only impermanent/unstable but is totally unreal — a collective delusion or illusion, resulting from us having forgotten that we are all already enlightened and are actually one big, unified/monistic, real Mind (variously called absolute reality, Buddha-nature, Tathagata-garbha, etc.). As such, it was/is possible for the many past Buddhas and Bodhisattvas to come and go between nirvana and samsara, and to create and destroy dualistic worlds (such as Pure Lands, where it is supposedly as easy as possible to attain nirvana), as they please. Attaining nirvana makes one like a God in this world.
There are two states of matter/energy existing right next to each other, which rarely interact (similar to dark matter vs. regular matter): one is more permanent and stable (nibbana), one is more impermanent and unstable (our reality, called samsara). Both samsara and nibbana are real, though living beings may misperceive or misunderstand them, and both have their pros and cons. The cons of living in samsara are things like old age, sickness, death, loss, pain, fear, vulnerability, etc., but the pros are that it can be fun, exciting, pleasurable, etc. Nibbana is basically the opposite of samsara: no old age, sickness, death, etc., but it is also very still and lacking pleasure or pain. In that they are quite different states of matter/energy, a person cannot easily come and go between them. The samsaric body must die before an enlightened mind (having extinguished kamma/karma and totally let go of this world) can transfer into a nibbanic state (called parinibbana), and nibbanic beings apparently cannot or do not transfer back into samsara. When the Buddha became enlightened, his mind could only see nibbana from this world, like looking across a river at the other shore. His samsaric body had to die for his mind to somehow join with or become nibbanic matter/energy. The Buddha was just a man who discovered something great (i.e., a way for the mind to apparently live forever without suffering). He was/is not some kind of God — when Theravada Buddhists “worship” the Buddha, they are typically trying to gain karmic merit for themselves by doing something good or noble, not trying to interact with the Buddha (because nibbanic beings do not interact with samsaric beings; they just set an example for samsaric beings to emulate) — and nibbana is not an absolute essence for samsara.