Bleach TYBW part 1 finale witnessed Ichigo learning a lot about himself and his lineage, as he discovered that he was not an ordinary Soul Reaper. While fans have already received hints about the same, even in the original anime, Bleach TYBW episode 12 finally confirmed Ichigo's lineage and how he was related to the series antagonist Yhwach.
Bleach: Thousand-Year Blood War has focused entirely on the Wandenreich Empire's attack on the Soul Society. With the Soul Society dealing with many casualties, Ichigo, along with some other Shinigami, was taken to the Soul King Palace to recover and train with Squad 0 members, during which Oetsu Nimaiya made Ichigo realize that he did not know who he was.
Bleach TYBW maps out a background of Ichigo's parents
Bleach: Thousand-Year Blood War part 1 finale focused on the backstory of Ichigo's parents as it showed fans how Isshin Shiba and Masaki Kurosaki came together. While Masaki was meant to get married to Ryuken Ishida for the prosperity of the Quincy bloodline, a fateful encounter with Isshin led her to fall in love with the Soul Reaper, leading them to get married and give birth to Ichigo.
Given how Masaki was a Quincy, she was inadvertently related to the Wandenreich King, Yhwach. Yhwach, as revealed by Isshin, is not just the Wandenreich King but also the Father of the Quincy, meaning that he created all Quincy.
Thus, Yhwach was related to all Quincy by their blood, meaning that Yhwach was the maternal forefather of Ichigo.
This is why when Yhwach realized that Ichigo's Quincy powers were awakening, he happened to refer to him as his "son born in the darkness." Unlike Pure Blood Quincy, Ichigo was born from the union of Masaki Kurosaki, who was part Quincy and part Hollow, and Isshin Shiba, who was part Soul Reaper and part Human.
How did Masaki Kurosaki lose her powers?
As revealed by Isshin, Masaki lost her Quincy powers due to an event called Holy Selection, caused by Yhwach. In Quincy folklore, there was a sealed king. According to that, the Quincy king, who was once sealed away, took 900 years to retrieve his pulse, 90 years to retrieve his sanity, and nine years to regain his power.
Nine years ago, when Yhwach started regaining his powers, he conducted the Holy Selection, through which he took the powers of those Quincys he considered impure and made them his own. Due to that, Masaki Kurosaki lost her powers on the day she was fighting Grand Fisher. She could not access her Blut Vene and thus lost the fight and passed away.
This is the same reason why Uryu's mother, Kanae Katagiri, who was already weak, also passed away.
Yhwach was able to do so because he was the one who created Quincy. They originated from him, which is why he is connected to them. This is the same reason Ichigo could locate Yhwach and instantly know that he was the enemy leader as soon as he entered the Soul Society.