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.
Williams himself wants to play in Barcelona and even agrees to receive a not too large salary (which will still be higher than in Bilbao) so that Barca can afford to have him on their roster. But as I already wrote at the moment, this is not the main thing, since if the compensation under the contract is large, then everything is in the hands of Bilbao and they can demand either the entire amount due for the premature termination of the contract or negotiate on the terms that suit them.