When I was 5 I wanted to be a chocolate chip cookie. I remember vividly driving up to a dumpster behind a big box store to find a single piece of cardboard big enough to make a me-sized cookie. Then I got all upset because everyone at school wanted to eat me 🤣
While a little arbitrary, we use "ability to produce viable offspring" as a metric of speciation. Two animals can bone and create an offspring, but that offspring has to have live gametes (egg/sperm) for the parents to be considered the same species.
My biggest issue with 3 was how the writers failed to build a good mental map for the audience of the tunnels. The whole 10 minute "chase" scene had me thoroughly confused as to how close the alien was to anyone else.
And you then spend longer verifying the information its given you than you would have spent just looking it up to begin with.