In 2018, there was an art trend of drawing the Doomguy teaching cute characters how to shoot guns - Mega Man, Inklings, Rapunzel, etc. About a year later, someone made a version featuring Isabelle from Animal Crossing.
The contrast of the ultra-violent space marine and the sweetly innocent administrative assistant proved popular, and only grew when fans realized the newest games in the series, Doom Eternal and Animal Crossing: New Horizons, would be releasing on the same day. Yes, the same thing that would later happen to Barbie and Oppenheimer.
Even the creators of both games got in on the fun, doing their best to hype up the other release, making this as official a crossover as it can be without being an official crossover. And that's why Doomguy and Isabelle are best friends.
I like the 'Doot' Revenant in the end credits.