The whole purpose of the Starship concept is that it makes transport from Earth to Mars and back again not just possible, but practical.
The first part of the concept relies upon the use of orbital refuelling to refill the Starship in LEO. This basically externalises the staging of the system and allows huge cargoes to be transported to the red planet. Think of it this way, if you are heading to a destination that is literally millions of miles from home, with no prospect of a rescue mission if things go wrong, do you want to be in a tiny capsule with just a few tonnes of supplies, or do you want a huge space ship around you, and 100 tonnes of gear with you? Starship allows this to happen thanks to it getting a fresh charge of fuel and oxidiser in orbit, effectively “resetting” the rocket equation at a position that allows a much lower energy transfer to occur.
Now you have a spacecraft with full tanks, just like on the launch pad, but already travelling at 8km/s, giving you a huge boost in what you are able to do. This new fuel allows you to boost out of Earth’s gravitational influence, get to Mars and slow down to be “captured” by it. The “belly flop” landing technique is yet another fuel saving method that uses the thin Martian atmosphere to put the brakes on, as it were, slowing the craft down by 90%, meaning that only a small amount of fuel is required to land the behemoth.
Now you are on the surface of Mars on a giant spaceship with empty tanks, which seems like a bad situation, but this is not the full story. Your Starship is not the first to land on Mars, in fact there have been several Starship landings at your landing zone over the last few years. These unmanned, robotic missions have paved the way for your arrival by establishing an energy source (solar or nuclear power), and assembling a fuel depot that uses local resources such as water ice and CO2 to synthesise methane and oxygen that has been stored back in the tanks of the first Starships to arrive.
This means that you already have the fuel required to return back to Earth ready for you before you even leave the launchpad on Earth. You are arriving at a pre-assembled Mars base, complete with power, fuel, and oxygen. Add to this your extra 100 tonnes of supplies, and you have a viable method of making the first human landing on another planet.
OK, so the domes and roads might be a little too much for the first robots to manage, but the first humans to land on Mars won’t be duplicating the conditions that the Apollo astronauts had to deal with on the moon. There will be a level of support and material available that greatly increases the odds of success.