If you are trying to select the person in the image, which is what I understood from your original thread, IMO there are better/easier tools to use than the magnetic lasso. For example, a simple click on the Select Subject button produced this result:
Where there are several subjects in an image, you can draw a rectangle around one of them using the Auto Selection tool and achieve the same result.
The Quick Selection brush is also, in my mind, a far better and easier tool to use. I haven't used a Magnetic Lasso in years.
I'm not clear about your question. Do you mean that after using the Select Object button (or Auto Selection tool) some of the background is selected? If so, you can modify the selection by using one of the other selection tools such as the Selection Brush or the Quick Selection brush. You can also use a mask to clean up the selection. In my following screenshot:
The Object Selection tool selected some of the car in the background
I selected the Quick Selection tool and used the Subtract option (you can also use the Alt key to subtract from normal mode)
The black car was easily removed but some roadway remains selected