First somebody made something up for some obscure reason, perhaps anticipation and boredom, but perhaps as professional propaganda to map the gullibility of humanity and to throw another wrench into the whole works for us.
Then a lot of somebodies thought it sounded good so they made up more stuff to go with it. (That's just the way people are.)
Some of all the untruths are based on some truth of some things that are real, such as that there are real alien races, that they have always visited Earth, and that they are from nearby worlds.
But that's all.
Almost everything you ever hear on Earth right now about alien life is inaccurate assumption, stolen bits of various similar materials, or complete and utter fabrication.
But don't let that make you think that is what the alien beings are like.
The alien beings who are visiting Earth are the professionals of their races, working here, together, to build a diplomatic relationship with our world.