Welcome to the Community, @Collier.
Normally, if you Write Checks to pay for existing bills, the two transactions won't be linked and the bill remains in an unpaid status. There is a couple of available solution to link your check to your bill depending on the situation that applies to your end.
You may refer to this article for the steps by step process to link your check to your bill: Bill shows as unpaid after writing a check in QuickBooks Desktop.
If you also want to learn how to use Online Bill Pay in QuickBooks Desktop, feel free to read this article: Use Online Bill Pay in QuickBooks Desktop.
Let me know if there's anything else I can help you with. Have a great and take care.