What do these words have in common: Sitcom, smog, brunch, muppet, blog, bionic, motel and cyborg?

They are what some people call portmanteau words, i.e. words made by combining parts of two or more other words.

Smog = smoke and fog; brunch = breakfast and lunch, and so on.

“Portmanteau word” is a term inspired by Lewis Carroll. When Humpty Dumpty is explaining “Jabberwocky” to Alice, he says that words like “slithy” are “like a portmanteau: there are two meanings packed up in one word.”

In the book, Humpty Dumpty explains to Alice words from Jabberwocky, saying, "Well, slithy means lithe and slimy …. You see it's like a portmanteau—there are two meanings packed up into one word." Carroll often used such words to a humorous effect in his work.

Wiki

blends

A portmanteau (plural: portmanteaux) or blend is a word or morpheme that fuses two or more words or parts of words to give a combined meaning.
https://en.wikipedia.org/wiki/Portmanteau