As the title states. Due to it being about synths, then maybe if you choose the BOS you get to kill them all? Railroad you get to free/help them all? Institute you get to enslave them all?

I guess is to early to question it, might be tied up to the MQ ending or not.

It can be a synth surface colony known from institute or not, eventually it has been established from the railroad, as its them who get the synths rushed out of the commonwealth.

Maybe Zimmer is there for some reasons given how many F3 references are around F4....

Beth does mention more Faction Quests in the description and that might refer to original factions rather than the three local ones.

If Railroad even has anything to do with it. They tend to prefer mind-wiping Synths, changing their faces and sending them off into the society.

As far as we know, these are Synths who fled the Institute in more distant past, got together and went to create their own community.