Terraforming Mars is impossible. Well… maybe not impossible but it would be highly improbable and would cost way more than it’s worth. Let me tell you why:
Mars has no magnetic field. This means cosmic rays and charged particles from the Sun are just striking the planet all the time. There’s ways to maybe stick something on the L1 Lagrange point between Mars and the Sun that could act as a kind of magnetic shield but that’s a little beyond our level of engineering at this point.
Mars isn’t massive enough to hold onto it’s atmosphere. For a planet to keep it’s atmosphere it needs to be large enough and the gasses need to be replenished. Right now if all the trees and green things stopped producing oxygen we’d have enough for maybe 1000 years. After that point the oxygen would have spilled out into space or would have been turned to CO2.
Nothing can grow in Martian soil. It’s littered with perchlorates which are toxic. But on the plus side we could take those and rip the molecules apart to make oxygen. So we’d need something to clean the soil, add the good bacteria to it as well as organic material to make the soil able to grow crops.
There’s not a lot of water meaning water would need to be brought from elsewhere.
Yeah. Mars isn’t going to be terraformed for a long time assuming it can be in the first place. You’re better off with pressure domes and people living underground.