Reaching new Milestones in Rainbow Six Extraction is how you unlock new gear, characters, and maps. The Milestone system acts almost like a battle pass and unlocks various items as you progress through it. Here is how you unlock Milestones in Rainbow Six Extraction.
The only way to unlock Milestones in the game is to gain experience. There are different ways to gain experience, however. It is important to understand how you can gain experience quickly and unlock the higher Milestones faster. The first way you can gain experience is by killing Archaeans. Each kill will get you a small amount of experience. You will gain more if you remain stealthy and kill Archaeans using their weak points.
The second way to earn experience is simply by completing missions in the various locations you can go to. Completing missions on any difficulty will earn you experience towards the operator you chose and the Milestones. Completing more missions in an area will earn you more experience than completing missions on higher difficulties.
Finally, you will gain experience by completing Studies. Studies are small objectives that you can complete during missions. They are often small tasks like killing certain Archaean types or getting stealth kills. Completing these will get you large amounts of experience that will go toward your Milestone progression.