If Leverkusen extends the contract of Xabi Alonso they will not let any other teams including Liverpool or Bayern Munich even think about their coach to hire Xabi Alonso for the next season. But the fact about Xabi Alonso and Leverkusen is they are going to play in the Champions League for the next season and if they have Xabi Alonso, they will be a strong team in this tournament. However, I think Leverkusen will also need to hire some players and get improve to make them ready to play against stronger teams like Real Madrid.
Sadly we don't have Leverkusen in the Champions League in this season, but I'm waiting for it until the next season.
Of course, because Xabi Alonso extends his contract with Bayer Leverkusen, Xabi won't move to any team. Liverpool and Bayern Munich must wait for the end of his contract, his contract will expire in June 2026. It means Xabi Alonso may move to a new team in the next 2 years if there is nothing change with him and Bayer Leverkusen. But if there is something change, Xabi can leave Bayer Leverkusen earlier.
It is true, Xabi Alonso will accompany Bayer Leverkusen to play in UCL for the next season. It is very interesting to see how Bayer Leverkusen performance in UCL. I'm sure Xabi Alonso will improve Leverkusen squad due to playing in a higher competition. In my opinion, Xabi Alonso needs a better attackers because current Leverkusen attackers look not really convincing.
For this season, Leverkusen played in UEL, dude. I think it is not too bad, Leverkusen can win UEL trophy first before they win UCL trophy for the next season.

Source: thesunTherefore, rather than hastily moving to another team, it is better for Xabi Alonso to prove himself with his squad in the Champions League next season. Of course we are very curious about Leverkusen's arrival in the next season and it will be increasingly difficult to predict who will be the next champion after Leverkusen is in this competition with the same squad and coach.
We must hope that the Champions League competition can be a proving ground for strong teams and with Leverkusen it will add more color and tension to every match. Therefore, it would be very regrettable if Xabi Alonso left sooner and it would be better if he remained Leverkusen coach so he could make other achievements. Moreover, this is Xabi Alonso's first season leading Leverkusen in Europe's highest competition so he cannot miss this.