When Nadine (Amber Heard) drops Joe (Gordon Cormier) off at the school before the vigil, the film Time Bandits (1981) is playing in the background, which is Terry Gilliam's depiction of the universal struggle between "good" and "evil".
(At 21:37) When Flagg (Alexander Skarsgård) says, "My name is Legion, for we are many," the last echo of the word "many" is the voice of Jamey Sheridan, who played Randall Flagg in the 1994 mini-series version of The Stand (1994).
The half-size replica of the Eiffel Tower can be seen from Flagg [Alexander Skarsgård)'s window. His room would have to be in Planet Hollywood Resort & Casino to have that particular view of it.
Ezra Miller (Trashcan Man) and Amber Heard (Nadine) both appeared in Justice League (2017)/Zack Snyder's Justice League (2021) as Barry Allen/The Flash and Mera, respectively.