T-Bills - pls help me to understand what happens to the principal investment upon their maturity date? Do I need to do anything or will the amount be credited back to my bank account by default?

Upon the maturity date of T-Bills, the principal investment amount is automatically credited back to your bank account by default. You don't need to take any action to receive the principal amount. The process is typically handled by the financial institution or brokerage firm through which you purchased the T-Bills. When the T-Bills mature, the principal amount, along with the interest earned, is paid out to the holder. If you hold T-Bills through a bank or brokerage account, the amount will be credited to that account.

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You can find out more about T-Bills in this article.

Credited back to bank account by default