Before we start talking about how our alien culture works, we need to work out what our aliens look like. For most on-screen science fiction, and a considerable amount of written sci-fi as well, the go-to alien shape is the bumpy-foreheaded human (or pointy eared human, or human painted blue).
Even this is still considered excessive effort for some, with Doctor Who’s Time Lords, Kaleds, Thals, and the people of the planet Sto all being indistinguishable from humans.
The reasons for this aren’t simply stinginess in the make-up department. Aliens need to do certain things. They need to be able to use tools, have conversations with the human characters, drive spaceships, serve drinks, sit at tables, all things that we’re used to seeing human shaped bodies do.
And the thing is, this is actually okay. Think about the dolphin, the shark and the prehistoric reptile the ichthyosaurus. They’re all roughly the same shape, despite being as related to each other as they are to you. But they all live in similar environments, facing similar challenges and filling a similar ecological niche.
It’s not completely outlandish to suggest that maybe an animal unrelated to us would find an evolutionary advantage in standing upright, developing opposable thumbs on their forelimbs to manipulate tools and having two eyes on the front of their head to allow for depth perception.
But while there may before perfectly sound reasons for making your aliens humanoid, that is y’know, boring.
This is why a lot of writers look towards wildlife, particularly insects and deep sea creatures, who live in the closest environments to alien planets we have access to on this one (those being really deep under the water, and exactly like our world only much, much bigger). Squid and octopodes are particularly popular models, with their tentacles doing a lot of the jobs we might expect of an opposable thumb.
Adrian Tchaikovsky uses both of these in his Children of Time/Children of Ruin books, where the “aliens” aren’t alien, but are in fact hyper-evolved Earth animals. Ken MacLeod’s Learning the World features a planet of bat-like aliens, and shows us all kinds of ways their shape dictates their culture.