Help to process the picture, the background turns white, please tell me the detailed PS processing steps, thank you very much !

Copy the layer and draw an edge along the outline on the new layer with the pen tool.

Close the outline, and ctrl+enter converts the pen path into a selection.

Ctrl+shift+i inverts the selection and sets the foreground color to white, and then alt+delete fills it.

If you need the reflection below, copy another layer of the doll, located below the original layer, freely transform, flip, adjust the transparency, and then gradually change it with a mask.