Can a researcher hire 100% themself with funds from H2020 project (or other call)? Once in some conference I heard that some guy was working as researcher (PI in different projects) at some university in Italy, but he had no official position. I understood that somehow, before entering this university he applied to an H2020 project (no ERC, StG, CoG, or AdV) that he got and that in the conditions he put in the budget 100% of his salary. So he kind of hired himself. Then he was always applying to H2020 (or other) calls in order to get continuity of his contract.

I wonder how this works.
It's quite normal for people to pay 100% of their salary (or the salaries of researchers, post docs, etc) out of H2020 project grants. I'm not following what you see as the problem. It just means that person spends 100% of their time on that research.