okay but can someone actually explains this whole box and presents thing to me?

It was the little shit's long winded way of saying that Itadori Yuji himself wouldn't change - like it's still Yuji in his body - but by using that aforementioned 'soul-swapping' ability, Kusakabe's skill or a chosen ability of his (simple domain) would be transferrable from his soul to Yuji's.

It's kinda like when Mahito learned black flash just through observation and turned into that burnt looking chicken tender thing.
Why they couldn't teach Yuji simple domain directly heaven knows, but I'll assume they were pressed for time.