The basic concept behind this is the value of color black ,it’s value is 0 in OPEN_CV.So black + anycolor= anycolor because value of black is 0.
Now suppose we have two images one is named img1 and other is img2. img2 contains a logo which we want to place on the img1. We create threshold and then the mask and mask_inv of img2,and also create roi of img1. Now we have to do two things to add the logo of img2 on img1. We create background of roi as img1_bg with help of : mask_inv,mask_inv will have two region one black and one white, in the white region we will put img1 part and leave black as it is-
img1_bg = cv2.bitwise_and(roi,roi,mask = mask_inv)
In your question you have used directly the mask of the img created
res = cv2.bitwise_and(img,img,mask = mask_img)
and in img2 we need to create the logo as foreground of roi ,
img2_fg = cv2.bitwise_and(img2,img2,mask = mask)
here we have used mask layer , the logo part of img2 gets filled in the white part of mask Now when we add both we get a perfect combined roi For full description and understanding visit: OPEN CV CODE FILES AND FULL DESCRIPTION