the white background is part of the image it self for the first 3 image but the svg is a vector graphics which you can think of it draw the image based on mathematical way and it represented in xml so you can open the file and edit it as if it was text file so the svg is just that shape the you see that why you see the real background which is some kind of pink
ofcourse other image could be transparent but the person who create it should do it based on his/her tool like photoshop or something
Nice job @tuhamworld! With regards to the last image, @justsomeone is correct with their explanation. As an extra challenge, can you think how you would give that image a white background colour as well?
Hint: You can do it without having to edit the SVG.