A few instances are ideologically driven. So lemmy.world and lemm.ee tend to be pretty straightforward instances politically slightly left of centre but there are others like lemmy.ml and hexbear that push Kremlin/CCP propaganda. Anyone can set up an instance and federate with the other instances. But instances can also de-federate with specific instances. For example most instances have de-federated from hexbear and threads so posts made to them won’t show up for users who use lemmy.world for example.