How do you see all the communities on an instant?

There's a "Communities" button at the top of the page (or in the top menu on mobile) that shows all communities on your current instance.

I'm not sure for your search question. It looks like they tried to implement that function in the general search tool (magnifier icon at the top/menu), but filtering by a specific community doesn't seem to work for me. There are still some bugs to work out on the site in general.