Are you asking why it is opaque to view electronic signatures on mobile phones? There may be no reason to remove the background color.

Electronic signature background removal:

Open the picture and open the electronic signature picture with Photoshop software. You can see that the background of the signature is white, not a transparent gray square.

Select the signature part of the picture. Using the quick selection tool, select the white area in the picture, right-click and select "Select Inversion" to select the signature part in the picture.

Right-click the selected part of the copied image and select Copy Image. In the layer area in the lower right corner, you can see that there are two layers, one of which has no background color. Turn off the display of the original image and you can see that the newly created layer has no background color.

Export the layer with the background color removed. The background color is transparent in PNG format and still white in JPG format.