It seems that Real Madrid are watching Bayern Munich's left-back. He is Alphonso Davies, 23 years old. His contract with Bayern Muchen expires this year, so they don't want to repeat the mistake of Kroos' move. Kroos moved on a free transfer. Munchen does not want Alphonso Davies to move on a free transfer and perhaps this will make Bayern Munich have to let him go this summer.
Real Madrid actually has 2 left-back (Mendy and Fran Garcia), they don't really need new left-backs. Real Madrid is watching Davies because he has a chance to leave Bayern Munich with a free transfer status. Since Davies is a top left-back, Real Madrid don't want to miss the chance to sign him for free. However, if Bayern Munich extends Davies's contract, I'm sure Real Madrid will prefer to stop watching him.
Real Madrid should focus on looking for a new center-back and right-back. In these positions, some players are more than 30 years old (all Real Madrid left-backs are 32 years old). Real Madrid need to plan to sign new players with young ages to replace the current players. Reece James, Alexander-Arnold, and Achraf Hakimi are good candidates for the next Real Madrid right-back.
Actually, even though Real Madrid already have 2 strong left backs, they are still interested in Alfonso Davis to build up star players. If there is a deal then the left back that Real Madrid currently has will definitely be for sale. because the quality of Alfonso Davis is still above Real Madrid's current left backs.