Apparently they come from a series of commercial for McDonald's Japan's post-5pm menu line-up, known as Yoru Mac. These specific character are a mother and a girlfriend, who don't appear in the same commercial. And have male love interests, obvious
The characters in these commercials were trending for various reasons, memes, fan-art, so fourth. Then basically some artists said "Okay but what if the mom from this one commercial, and the girlfriend from this other commercial, were a thing?", and so we get fan-art such as this. The power of the internet haha
^
You don't know how hard it is to tear Seseren away from doing fan animations of mobile gacha characters. I can't recall him ever doing art consistently for a seasonal anime.
She specifically says she's not an old guy (or a pervy old guy, don't remember which), meaning she's talking about the fantasy of having a sexy secretary here, not women in general. Also I don't believe Nami and Vivi had a relationship, but Nami sure looked at Vivi like she loved her.