What happens when a T-bill matures?

Excuse me, could you please explain what takes place when a Treasury bill, also known as a T-bill, reaches its maturity date? I'm curious about the process and what happens to the investor's funds at that point. Additionally, I'd like to know if there are any specific steps or procedures that need to be followed, and whether the investor has any options or decisions to make once the T-bill matures. Thank you in advance for your clarification.