Its easy to write characters and build characters when they based on the familiar. On beings we encounter everyday...humans, to be exact. You tend to write about what you know.
What about when you are choosing to write about an alien or alien race? How do you decide about traits, characteristics, cultures, etc?
I went in search of some resources to assist with just those things. The first site I found made me smile by the title of it. Forgive me, but I'm not a believer in alien abductions. So the title "Malevolant Alien Abduction Research" struck me as humorous. That particular site is apparently defunct.
The next, and most comprehensive is www.aliens.fandom.com. While not exactly and A-Z encyclopedia, it does offer useful details which you might find invaluable in creating your own alien species.
Wikipedia provides a list from different science fiction universe. Examining these may allow you to develop a greater variety in your literary world and solar systems.
The list is extensive, with pages for each letter of the alphabet.