How did the Alien

Movie magic to service the plot.

Xenomorphs aren't real and thus the writers can make them do whatever the fuck they want or need of them.

This has been discussed many-many times on this board already.

My answer is in this comment below:

https://moviechat.org/tt0078748/Alien/5e51e3fdd7d6b221a719f048/3-Things-that-dont-make-sense?reply=5f329a538f206a2193af0351

It's worth reading the subsequent replies as well! Please do come back if you have anything to add to this.