It's a combination of everthing he found and what was already there

How because in the story it was implied that the one piece was joyboys not rogers and then the one piece existed before Roger

The one piece is a combination. Roger said he left everything there and it was dubbed the One Piece

Didn’t Roger go out to sea to find it

No

He created it

Ok

One Piece was the name given by the civilians to Rogers treasure...

Since Roger was the only one to reach Raftel and find Joyboys Treasure there, just makes sense that for the civilians that the One Piece is a combination of those, since they dont know about Joyboy for them it only exist Rogers Treasure, the One Piece