The following contains spoilers from BLEACH: Thousand-Year Blood War Part 2 – The Separation, Episode 1, "The Last 9 Days", available to stream on Disney+.
The first episode of the second cours of BLEACH: Thousand-Year Blood War established some of the major narrative elements that are going to be significant this season, highlighting the Shinigami in their preparation for the upcoming war against the returning Vandenreich in nine days' time, as well as the strides taken by the Quincy King to ensure the longevity of their oppression of the various non-Quincies in the BLEACH universe; Shinigami in particular.
With this first episode, Ichigo embarks on a new training regiment that stands to make him so powerful, he won't be able to return to the world of the living; however, despite that development, it isn't even the most significant aspect of this initial episode BLEACH: Thousand-Year BLood War Part 2 – The Separation. The biggest development is the confirmation of Uryū Ishida's complete defection from the Shinigami side of the war, and the reasons underlying his betrayal.
The Event
The first 90 seconds of the first episode of BLEACH: Thousand-Year Blood War Part 2 are filled with the journey of a certain being from conception to birth. The atmosphere of this scene is calm but eerie at first, until the fully-formed baby is then raised up high by countless shadowy hands. The scene zooms in and out, scanning the various hands, each of which have a pale blue light in their palms. As the baby is held up, almost like Rafiki holds up Simba in the beginning of the first Lion King movie to be gazed upon by all his future subjects, this baby is held up by these countless, shadowy hands while the syllables "Yu~Ha~Ba~Ha" are chanted over and over. At some point, the pale blue lights beam with brightness and create a huge light in the midst of that dreary darkness; however, in the blink of an eye, this benign-looking light is replaced with the eerie red glow of countless red (umbilical) cords connecting to the baby, now floating above the masses.
As these connections continue, the baby rapidly develops into childhood, before the silhouette of the materialized form of Tensa Zangetsu develops from that child. His eyes open, revealing a steely gaze as the sight transitions from young "Tensa Zangetsu" to Yhwach himself. This first 90 seconds communicates the event known as the Auswahlen, a moment in history in which the Quincy King, who as a baby had no ability and no power, has the blessings that he imparted upon his people returned to him en masse. Whenever people touched baby Yhwach, they would be imbued with powers or abilities that made up for what they lacked; however, upon death, this strength would return to him. The chanting heard in this scene is what Yhwach eventually took on as his own name, although, it was the name of the god that the people around him worshipped. Evidently, the four syllables of "Yhwach" as it is pronounced in Japanese reflect the tetragrammaton, the four Hebrew letters used to invoke the true name of God.
The Successor
The ritual that Uryu is required to undergo prior to his full inclusion in Yhwach's forces is one in which the blood of the Quincy King is consumed, enabling him to assign a Schrift – a letter which serves as the very basis of their role and abilities as Sternritters. Before continuing, Uryu asks Yhwach why he was selected over anyone else, and the answer lies in the mechanics that are behind Uryu's survival of the Auswahlen – the only Quincy in history ever to do so. Uryu's reasons for betraying Ichigo and the Shinigami lie in the diary left behind by his grandfather, Soken Ishida. From the insights written therein, Uryu came to the conclusion that Shinigami and Quincy can never get along. With his resolve established, Uryu drinks Yhwach's blood and is assigned the schrift "A", the same as Yhwach himself.
Yhwach's decision to have Uryu as his successor lies in the reasons for Uryu's survival of the event that stripped many Quincies of their powers, and some, their lives. When he addresses his soldiers to tell them of his decision to appoint Uryu, the reactions are varied, but the palpable rage of the majority can be seen. Quincies like Bazz-B and Bambietta Basterbine have a hard time accepting the news, believing that the person who was meant to be Yhwach's successor is his current right-hand man, Jugram Haschwalth, who seems completely unfazed by the developments. As the Shinigami prepare for the next invasion of the Quincies expected in 9 days time, Yhwach seems to have amassed all the necessary pieces to ensure his victory.