Hazbin Hotel and Helluva Boss share a universe with Brandon Rogers' videos
We already know that Hazbin Hotel and Helluva Boss take place in the same Hell. We also know that most of Brandon Rogers' videos take place in a shared universe. Brandon Rogers is co-writer of Helluva Boss, and voice of main character, Blitzo. A character that looks just like Brandon's character Flint Dicker appears in the Helluva Boss pilot. It seems very likely that they could share a universe. Maybe people turn into cartoons when they enter Hell. The living world is live action, but is shown through an animated filter in Helluva Boss, or maybe some other explanation. Japanese Girlfriend isn't faking being Asian...
...she's faking being Japanese specifically. She's still Asian, but from a different Asian culture, so the joke still works.- Considering that Brandon himself is part Filipino, this likely isn't far off.
The kid pretending to be Grandpa at the end of "Grandpa HATES Disneyland" is Timmy
Timmy is shown to have a very active imagination and pretends to be many different characters while playing in "A Day at the Park", including Grandpa.