FLemmingO

Browsing All shows posts from all communities that have previously been interacted with by a user from your own instance. So if someone from Lemmy.fmhy.ml subscribes to an obscure community on some other instance, that community is added to your All view, but it wouldn’t show up before then.

Rule 4: No self promotion of any kind. This community is not for advertisements.

Yup! Those websites are the instances which are the infrastructure and interface by which you can access communities - each of which is hosted on a specific instance. An instance is essentially the collection of all the communities it hosts and all of the users signed up through that instance.