Asteroid Mining is a mechanic added in the Wanwood Update.
When you buy a refinery module for a station, you will see it has a hangar. Here you can load off resources mined with the excavator.
To mine with an excavator, go up to an asteroid and click LMB. When you are done mining return to the refinery and land. A GUI will show up which will allow you to unload your resources
Refinery Upgrades The refinery has 3 upgrades. Each of which increase the amount of RUs received from asteroid mining. The RUs will be split equally across the entire team, meaning a smaller team will get more per person.