Santander Banco, one of Spain's and the world's largest banks, purchased a significant portion of Citigroup's auto loan portfolio. Santander did not buy all of Citi's auto loans, but your loan is apparently one in the portfolio it did buy.
Normally, when a loan is bought, the original servicer notifies the borrower as a matter of common courtesy. It tells you the loan was sold, who your new lender is, and where you will now be sending payments. In your case, it is understandable you were a taken aback by being contacted out of the blue by a party you have never heard of. After all, who in the US outside of the banking business has ever heard of Santander? Still, Citi and Santander's rude behavior does not mean anything improper occurred.
It was not illegal for Citi to sell your loan. The terms of your loan did not change when Citi assigned or sold the loan the Santander. Santander assumed all terms and conditions of your Citi loan, and it would be illegal for Santander to change your interest rate or the monthly payment required unilaterally. You stated the loan was modified, but I am not clear what you meant by that, and which terms or conditions changed.
Customarily, when a new creditor assumes a loan, it does not ask the debtor to sign a new or agreement. One of the reasons it is buying a loan is because the loan's history, risks, and expected return are known or easy to project. It baffles me Santander would risk losing you (and presumably other customers) by asking you to sign new loan agreements, which will almost certainly prompt many customers to shop around for better rates and terms.
What I am about to write is purely speculation, but perhaps the terms of your Citi loan are such that Santander believes it cannot make money on your loan, and would not mind losing you if you take your business elsewhere.
If you do not wish to do business with Santander, refinance your vehicle loan. Refinancing may not make sense if you are already years into the loan, because you will have paid off most of the interest on the loan at that time.
Alternatively, refuse to sign Santander's agreement. You are under no obligation to say yes to Santander's request to accept different terms and conditions.
I encourage you to read some useful information about auto loans that will help you compare and contrast terms and conditions of vehicle loans.
I hope this information helps you Find. Learn & Save.