This is exactly what was addressed when Yuji used his Domain for the first time and frankly, it couldn't be explained better. In Jujutsu Kaisen chapter 265, Yuji Itadori used Domain Expansion for ...

In the case of Yuji Itadori, he was able to swap his soul with Kusakabe, and that is how he ended up learning the power of Simple Domain. In JJK chapter 258, Yuji pulled out the ability called New ...