My loan was bought by another mortgage company in 2024, what is the principal balance of the original loan for 2025? Or is it $0?

If another company bought the first loan, the 2025 balance should be 0 because the balance was moved to the new mortgage company.  However, as you are entering your mortgage interest in TurboTax, the outstanding balance that the program asks for is the balance as of January 1, 2024, not the balance as of January 1, 2025.  You need to enter the balance as it is shown on both Forms 1098 that you  received from each mortgage company.   

You need to enter the balance as of the start of the year to see if there are limits to how much you interest you can deduct.   For more information see the below TurboTax help article on mortgage interest.  

Deducting Mortgage Interest FAQs