Not everything is a crossover, though - one particularly fun One Piece easter egg involves a character named Pandaman who appears in every episode of the lengthy series. Some references don't have a seemingly obvious connection - in fact, some anime even reference Western media, like when JJ Leroy from Yuri!!! on ICE appears dressed like Eric Cartman. In many cases, a studio like Kyoto Animation, Madhouse, Shaft or Trigger will include references to their other works - like when Kyuubey of Puella Magi Madoka Magica appears on a sign in Nisemonogatari. Sometimes, the connection isn't the studio, it's the original creator - the manga artists behind Hunter X Hunterand Sailor Moon are an IRL married couple, so it's not surprising to find crossovers.
The most common type of easter egg is a reference to another series. The more you watch anime, the more anime easter eggs you'll start to notice. Some are obvious, and some are subtle enough to escape even the most eagle-eyed anime fan.
