How does a galaxy differ from a constellation?
Galaxies are millions of times further away from us than stars in constellations, hence they look like a single object.
If you're in a forest, you can see individual trees, you can even record a pattern of trees from your position.
Now if you're in a meadow and see a distant mountain with a forest on it, you can't tell apart the trees it's just a forest. A constellation is the trees whereas the galaxy is the (distant) forest.
A factor of one million in distance will do that to you!