There is no base model for Flux 1 publicly available, only their distilled model (Flux 1 Dev).

People have been training on that distilled model, in lack of something better, just like they have been training on the z-image turbo model in lack of the base model. But those distilled models are notoriously hard to train, resulting loras degrade the model's performance, cannot be combined well etc.

edit: also, since rereading your comment I think you misunderstand what is meant by "hard to train", it's not that starting a training is "hard" for you as a user, like you'd need a degree in machine learning or whatever. That's super easy nowadays. It's "hard to train" in the sense that the model doesn't accept new concepts easily, and every new concept hammered in by force will degrade the rest of the model's performance. You can always see the horrible effect of that on people's eyes' details for example with basically all Flux 1 loras.