You can indeed call Murray Bauman's (Brett Gelman) revealed phone number (618-625-8313). It's a real number that leads to a very real fake answering machine message, where the character yells at you for calling.
The person who plays younger Max in the flashback is played by Sadie Sink's younger sister Jacey Sink.
While Steve (Joe Keery) and Robin (Maya Hawke) are tied up and having a heart-to-heart on the floor, the musical cue that plays is from Day of the Dead (1985) - a film the gang all snuck in to see in Chapter One: Suzie, Do You Copy? (2019).
The band uniforms that the Hawkins marching band is wearing were purchased from the Cheney Kansas High School. The original logo on the front of the uniforms was changed from saying Cheney to just stars.
The final sequence in this episode is underlined with music by Philip Glass.
He also wrote the music for the experimental film Koyaanisqatsi (1982) which became a cult-classic.