Williams is a good and valuable player and I'm sure Bilbao wants to make him stay however he can easily leave the team if he gets a good offer from the other team and this makes the fans of Bilbao angry because they can't keep their superstar player and Williams is very close to join Barcelona and play there for the next season. Even Williams himself likes to play for a team like Barcelona and this made everything harder for Bilbao to keep him there.
His performance was good enough while playing for Spain to make other teams send him an offer too.
Williams' contract runs until 2027, so Bilbao could theoretically keep him for another 3 seasons, but I don't see any information anywhere about what the release clause is in this contract. If it is 50-60 million or something like that, then Barcelona can simply buy him out without the consent of Bilbao, but if there is a serious amount, then the negotiations will be long and Bilbao will try to maximize their earnings.
I think Williams' value has increased after Euro 2024. Williams played an important role in the Spanish team's championship. If Barcelona wants to buy Williams, they must immediately pay 50 to 60 million euros in contract breaking fees, because Bilbao does not accept installment payments. Barcelona's cash flow is very difficult and spending such a large amount of money immediately is not easy. So it's very difficult for them to get Williams.
Not to mention that other big clubs in the Premier League are also paying attention to Williams. Due to financial constraints, Barcelona cannot compete with Premier League clubs. Barcelona should focus on developing the club's young players. Because Williams is still a young player and his future is still uncertain, Barcelona should not participate in this bet.