Asteroids (in our belt) are the most common source of raw material. They can be mined and transported more easily than a similar volume of mass extracted from a planet. Asteroids are very valuable and small ones can contain billions of tons of raw material. For this reason, individual prospectors, small and large development corporations are keen to profit from resources like nickel, iron, platinum, palladium, rhodium, ruthenium, iridium, and osmium. Water ice and ammonia are also useful.
In 2012, a company called Planetary Resources was formed to extract valuable minerals from asteroids and bring them back to Earth. This was backed by Google executives as well as James Cameron. They estimated a 900m wide asteroid contains ninety million tons of platinum in its core, worth $5.4 trillion, and a 30m asteroid could be worth $25 to $50 billion. An asteroid the size of Massachusetts called 16 Psyche is made entirely of iron and nickel and is worth about 10 quintillion dollars, or 70,000 times the global economy as at 2021.
A mining base could be established in the asteroid belt. A permanent station will maintain, resupply, and support future missions. Ceres, the largest of the objects in the belt, might make an ideal base of operations. Ceres is a dwarf planet, like Pluto, about a quarter of the size of the moon, with no atmosphere and little gravity. However, it contains one-third of the total mass of the entire asteroid belt. Given its weak gravity, it might make an ideal space station, as rockets would easily be able to land and leave the asteroid, which are important factors in building a spaceport (or moon base). Many asteroids, like Ceres, contain ice, which could be processed to extract hydrogen and oxygen for fuel.
Asteroids may be collected and moved as a whole towards central processing locations. Gravity tugs can be used to pull them. Small ships may land on the surface and construct thrusters and processors on the surface from material mined there, turning the asteroid itself into a ship which flies to its destination. They may also be broken up into smaller sizes to make mining and collection more manageable.
NASA's Asteroid Redirect Mission (ARM) aims to retrieve asteroid boulders from space. First, an unmanned probe would be sent into deep space to intercept an asteroid that has been evaluated by Earth-based telescopes. After a surface survey it would land and use hooks to grab onto a large boulder. The probe then would blast off and head to the moon, dragging the object by a tether. At that point, a manned mission would leave Earth, using the SLS rocket with the Orion module. The module would dock with the robotic probe as they both orbit the moon. Astronauts would leave the Orion, access the probe, and extract samples for analysis. Finally, the Orion space module would separate from the robotic probe and head back to Earth.
To move a 3 billion ton iron meteoroid from the asteroid belt to a parking orbit around Earth would require about 8.4 x 1018 J. A mass driver could be used to transport the mountain of metal. Using electromagnetic forces, small pieces of the asteroid could be flung from the driver as reaction mass, propelling the giant mother lode to Earth.
An effective way of performing large-scale mining operations such as mining an entire moon or asteroid belt would be by Von Neumann probes, taking advantage of their exponential growth. They could move on towards neighboring systems forever, seeking out raw materials (extracted from asteroids, moons, gas giants, etc.) making them the most efficient forms of mining operations possible.
Another possibility is that large-scale mining operations on other worlds will be for the benefit of space colonies, not for the earth. Colonists will extract the metals and minerals for their own use, since it might cost too much to transport them to earth. Mining operations in the asteroid belt would become economic only when we have self-sustaining colonies that can use these raw materials themselves.
Working-class humans in the The Expanse that mine asteroids are called the "belters".