I work on a MacBook, so the shortcuts mentioned below use macOS keys.

First, open your image in Photoshop.

Press Cmd + J to duplicate the original layer. This keeps your original image safe while you work.

Go to Image → Calculations.

For both Source 1 and Source 2, set the Layer to Background and choose the Gray channel.

Change the Blending Mode to Multiply.

Set Result to Selection, then click OK.

You will now get a highlight selection.

With the selection active, go to Layer → New Adjustment Layer → Curves. Adjust the curve slightly if you want to refine the highlights.

To extract shadows, go back to Image → Calculations again.

For both Source 1 and Source 2, set the Layer to Background and choose the Gray channel, but this time check Invert for both Source 1 and Source 2.

Change the Blending Mode to Multiply.

Click OK to create a shadow selection.

With the shadow selection active, go to Layer → New Adjustment Layer → Curves. Adjust the curve as needed to deepen or soften the shadows.