Are these generic sims based on someone or on some past characters?

They were part of the rebranding in 2019. I don't think they are based on any real people or previous characters from the sims franchise. They are available on the Maxis gallery with written backstories and you often see them when you start CAS.

I really like Mia Hayes, the blonde girl with the blue beanie. She is a freelance artist and a programmer so she was great to play with and try out the new freelance careers that they added back then.