how does Hubble telescope work ?

But... they do. Andromeda is outiside the galactic plane of the Milky Way, so we have a reasonably good view of it, but there's still a few of the Milky Way's stars in the photo. (And, the Hubble Imaging Team can 'remove' some of the objects from the image, if necessary - a 'super-photoshopping' of the image...)

Possibly the other answer is correct. More likely there is usually a few magnitude 3 though 10 stars in the foreground of Andromeda and it is easy to just think they are bright spots in the Andromeda galaxy. We can tell that some are not part of Andromeda by comparing with a photo several months earlier or later. Magnitude 11 though 24, in the foreground, change the appearance of Andromeda very little.

plants and suns are so far away they are like tiny dots and do not block out the images of distant distant objects....

Because they aren't that dense. There is dust between your eyes and your monitor but you can still see it clearly!