The Endless, Bless be He, formed the worlds below with the letter י. The Light came from Keter that doesn’t have any form but implied with the tip of the letter י.
The letters יה created the two worlds. The ה created this world and the letter י created the world to come. As it said in Isaiah 26:6 “with יה , he forms the worlds”.
When the Light of creation removed himself from the middle point in order to reveal the vessels, the letter י and ה, got revealed. The supernal Light did not stop the flow of Light to the lowers and from that came out the four levels of Chokmah and Binah, Tiferet (Z”A) and Malchut.
This is the secret of “ב יה שמו”, which are the special letters at the top of six columns of the Torah.