Colonising Mars with a million people is a huge challenge on its own, but what would it entail?
Mars is the closest planet to Earth in our solar system, even though, in its current state, it wouldn't support life. That doesn't mean we can't start inhabiting the red planet now though. Mars has a quite pleasant temperature of 35oC in the summer, but a -46oC in the winter is bitterly cold! That is not necessarily uninhabitable, as there are many places on earth that exceed these temperatures. Also, a full day on Mars takes only 40 minutes longer than a day on Earth.
But, the biggest challenge we face if we want to inhabit Mars is a lack of oxygen and freshwater. There is water on Mars's polar caps, but it is frozen and is only a very small amount in comparison to that of Earth.
Leaders in planning the Mars colonisation would be someone like Elon Musk, who has already been working on effective plans to start colonising Mars. One of which is to create vast climatization domes on the surface of Mars, which we can use until we completely transform the natural atmosphere of Mars to one similar to that of Earth.
Take a look at the video below by YouTube channel, Second Thought, on What Would A Million Person Mars Colony Look Like?