If God is immutable, how does the hypostatic union work?

According to Trinitarian theology as held by the Catholic and Orthodox Churches, God is 3 persons/hypostasis in 1 essence/nature/substance and one attribute of God is his immutability ([*c.f.* *Summa Theologica* Ia Q9 A1](https://www.newadvent.org/summa/1009.htm)) yet it is also the case that Christ is both true God and true man. This latter doctrine is called the hypostatic union referring to the two natures (divine and human) present in one person (hypostasis). How are these doctrines both held as true?