Moon is Earth's only natural satellite. It is the fifth largest satellite in the Sol System. Moon rotates synchronous so one side always faces Earth. The formation of Moon is theorized that a Mars-sized object in the early Sol System impacted with Proto-Earth. Moon has a massive abundance of resources such as Helium-3 that later becomes the propulsion for long-haul spacecrafts going to Mars and the Outer Sol System
Human exploration of the Moon began first with probes from then Soviet Union(present-day Russia) and United States and started off with the Space Race. The Americans was the first that landed there and walk on the Moon on Apollo 11 mission with Neil Armstrong to be the first one to walk on the Moon. 1972 was the final year in a long period that humans landed on the Moon. Since then, probes and rovers has been sent to explore the Moon by various countries on Earth.
In the science-fiction story, permament colonization began in the late 2020s and lunar bases was built, a shipyard to construct colonyships to the Outer Sol System and helium-3 refinaries to use as propulsion for fusion drive on the colonyships and other ships aswell. This wasn't finished until mid 2030s. Around the same time in the 2030s it was also made large-scale ice mines and then made into liquid water. Major factories was built outside the capital Tycho City.
Humans began colonizing the Moon in mid 2020s and lunar bases was constructed in late 2020s and also Helium-3 refinaries was begun to construct to mine after this resource to use as propulsion on colonyships and other transport-vessels. There is large deposits of Helium-3 on the Moon compared to Earth. With this, fusion drive was built to fit on colonyships that was also built on the Moon to go to Mars and the Outer Sol System. This was up and running in late 2030s and transporting colonyships to Mars from the Moon began in early 2040s
Mining after ice on the Moon is also something that was necessary to permamently settle on the Moon due to higher population on Earth and water resources was draining out on Earth.
Between 2060s and further out in late 21st century, Earth experienced a near extinction-level event. Most people were relocated to Mars but it was also people relocated to the Moon. Tycho City was expanded so much that it could withstand a population above 2 million people. Late in the 2080s, The Moon had a total population of nearly 2.5 million. Earth was not able to send much people to the Moon compared to Mars as at the time, settlements on the Moon was not expanded enough. Mars had already massive settlements to host billions