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.
Use the Foreground adjustment brush to select the interior (top left, brush strokes; top right, resulting selection), then the Matte adjustment brush to paint along the edge and individual hairs (bottom left). Use the Preview options to inspect the refined selection (bottom right).
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.