Where did constellationscome from?

Answer 1

Many of the currently-used constellations were invented in ancient Greece; others were added a few centuries ago, mainly by Europeans. In 1922, the IAU defined the current constellations, including the official limits between the constellations. You can find out more details in the Wikipedia article on "Constellations", section "History".

Answer 2

They come from people's imagination. A constellation contains a large number of stars and other cosmic objects which have absolutely no astrophysical relationship with one another. It is simply that they are located in such a direction, and are so far away, that they appear to move together. Furthermore, most constellations require a huge leap of imagination to go from a collection of stars to the image that the constellation is meant to depict.