#there are also a lot of parallels (connections) that could be drawn between yuuji and sukuna through such association with jizō...

[megyulmi]

Why Yuuji’s domain is a manifestation of his desire to save Megumi:

The hand symbol he used to open his domain is similar to the Mudra associated with Bodhisattva Kṣitigarbha, who is known as Bodhisattva Jizō (地蔵) in Japanese Buddhism.

In the common Japanese tradition, Bodhisattva Jizō is portrayed as the protector of the souls of children, who are condemned to stack piles of stones vainly on the banks of the Sanzu River (a mythological river in Japanese Buddhist tradition that one must cross to reach the afterlife), as these towers are repeatedly toppled. The legend has various versions. In one of the versions, the oni (demons) wreck the stone piles and torment the children, and the children seek haven with Bodhisattva Jizō who hides them inside his garment and comforts them. In another version, when the children pile stones at the 'Children’s Riverbed Hell', winds and flames are the agents knocking down the stone tower, and the flames reduce the children into cremated bones, to be revived back to whole by Bodhisattva Jizō.

It would not be an exaggeration to draw a comparison between the legend and the dynamic between Yuuji, Megumi and Sukuna: Megumi is one of those children at the riverbank, subjected to endless torment; Sukuna is the demon, the source of his torment; and Yuuji is the Bodhisattva, who has vowed to free those children from their torment, even at the cost of his own liberation (i.e. achieving Buddhahood).

We know that Domain Expansion is achieved by expanding one’s innate domain with cursed energy while using a barrier to construct it inside a separate space and that innate domain reflects one’s mind and soul. Saving Megumi has been Yuuji’s sole driving force, therefore it would not be an exaggeration either to assume that how he manifested his domain (i.e. the hand symbol for expanding it) is a reflection of his desire to save Megumi.