Once a selection has been created, you can refine its edges to ensure your selection is as accurate as needed.
For very fine selection, e.g. of hair against a coloured background, you can use an adjustment brush as part of the refinement to 'brush-in' fine detail.
Brush size can be adjusted mid-stroke from the context toolbar.
To refine pixel selection edges:
With a selection in place, from the Selections Persona, select Refine Selection Tool.
If you wish to adjust the selection edges by painting with the adjustment brush, set a Brush width that encompasses the detail to be included in the selection, then drag over the detail (e.g., hair) next to the selection edge.