they are displaying '666' using their fingers, like a gang …

they are displaying '666' using their fingers, like a gang sign, to signal that they're in the group. the bible warns of this happening, where the 'mark(symbol) of the beast' would be 666. the beast is the Mind Flayer (the hivemind), among other things...
https://twetch.app/t/514d47b461f3d8ab5ea61fb243247f830a9d99b0fd1f36a2e9ff1583dc30cdae