Writer and director Daniel Kokotajlo was himself a Jehovah's Witness for some years and raised in the religion.
"Bethel" is mentioned in the film. This is the HQ of the Watchtower Society which runs the JWs.
The disused Kingdom Hall was about to be converted into a café, so it was used only for exterior shots. The interiors were shot in a Lodge of the same shape and size as a Kingdom Hall which local Freemasons were happy to lend for the film.
As the girls discuss in the car that the cross is a pagan symbol. Unlike other Christian organisations, JWs do not accept the cross, but believe Jesus was put on a stake. They also refuse to use the word "church" for similar reasons, hence "kingdom hall" on their buildings.
Jehovah's Witnesses are not allowed to celebrate birthdays, hence there is no celebration in the film.