It’s not practical at this point. I wouldn’t say it’s completely impossible to colonize Mars.
Mars is essentially a desert. It could be used to farm metals and minerals, but they would need some sort of greenhouse to grow crops. Right now there’s no way to make it sustainable.
They haven’t found huge or or silver deposits, but we haven’t dug very far. Perhaps in 100 years or 1000 years we will have some massive shortage on earth of a certain mineral or element, and they’re just happens to be tons of it on Mars.
It sounded like Elon Musk wanted to make some sort of resort on Mars, and people could pay however much money like gazillion dollars to go and stay on Mars.
The biggest problem with colonizing Mars is obviously the distance. Even if they can harvest a wide range of raw materials there and use them for building, and even if they are able to grow crops and animals for meat and fish, they will need to transport a lot of equipment, and a lot of specialized materials such as medicines. To harvest raw materials on Mars will first take a lot of mining, refining, and manufacturing equipment, with a whole spectrum of equipment for each type of resource. The amount of material from Earth needed to bootstrap an actual city would be immense, and then there's still a basic question if they could ever operate in the black, or would always be dependent on Earth continuing to pour money into the Martian settlement.
The other biggest problem is the low gravity and the effect it will have on health long term, especially for any children born there. You'd pretty much have to count on some kind of future medical technology to make the body maintain its muscle mass and bone density in the light gravity. The chance of monstrous side effects from these treatments is obviously high.
As far as the moon goes, it is much closer, so people could go there to work, and then come back. The gravity is so much lighter that it's obvious that people can't live in it long term. People would to there for a period of time then come back to Earth.
The moon has a poorer range of resources than Mars, so you'd have to transport even more equipment and supplies there to sustain a population. However, I think it is possible that some kind of economically useful activity could be done on the moon. If nothing else, scientific research will justify some spending, and there will be a market for tourism.
I expect that Elon will land men on the moon and build some kind of facility there. It remains to be seen how much he can scale that up.
Furthermore, Elon still wants to go to Mars. I expect he will get there in 10 years or so, but it's hard to say if he can build more than a small initial manned base.