How do underground-Idol fans know what they have to do during performance?

Hello!

During my first trip to Japan I stumbled into my first little underground idol concert. I had no idea what was going on, but I had the time of my life :D
On my second trip I went to a concert of the same idol-group, and I was wondering how the fans know or establish their "performance" during the concert. Like, in each song there are different moves the fans do, at one point everyone grabs the hand of their neighbors, at one point everybody jumps to the right, at one point everybody gets really low etc. or moments where everybody shouts some stuff that is definitely not the lyrics (my japanese sucks, I couldn't make out what they said lol). It seemed to rather complement the song's lyrics.

It seems very coordinated and goes way above my western rock-concert experience of a front-man saying "open the moshpit" and "everybody get low". It seems like to each song there is a certain fan-choreography that everybody knows while the idols sing their song. And I wonder, where do they get to know this? Are there japanese forums where they discuss to each song what to do? Tutorial videos? Any japanese search queries to find stuff like this? Or do they just go to so many concerts that they learn that stuff while on concert?
A quick dilettantish serach of mine for this kind of stuff didn't lead to anything really.

The idolgroup in question is relatively small with around 1600 monthly listeners on spotifiy.

Thank you!