You can see content from every instance your instance is federated with (you can see a full list if you click on the instances link at the bottom of the page). You can see content from Lemmy and kbin (which is sorta like Lemmy but has some slight differences), and you'll also be able to see content from Mastodon users who tag communities you're in. I'm on sopuli.xyz for example, but here I am commenting on your post (how cool is that??)
On the "front page" of your instance you can choose if you'll see communities you're subscribed to, or only local communities, or all communities (and you can change what the default is from the settings in the upper right corner):
My default is "All" with scaled sorting (that little question mark will tell you about the different sorting options).
I think Peertube isn't federated with Lemmy at all, but as I said eg. Mastodon and kbin are.