Heh, I read your comment using Voyager on Android. Yeah, makes it easier for me to check how things are done on your phone that we happen to have the same telephone client app.

Probably the easiest way to grasp these things is to browse Lemmy on a full-fledged computer. Voyager can do everything the interface made by Lemmy's developers can do. (Plus, Voyager has a sensible way of reading your own private messages, while I have no idea how to return to them on the "official" web interface)
Since a phone has a cramped screen, all kinds of hocus-pocus needs to be done to fit the same features on the small screen. So, use it on a big screen of a computer and you'll get things much better. So, here goes:

Now, open a web browser, write "lemmy.world" on the address bar and then click "Login" in the upper right corner. Write "buttnugget" in the field "Email or Username" and your password in, well, wherever you feel it would fit well.

Now you're logged in in the same instance you always use with Voyager.

Press "Local" here, and you will see content only on communities hosted on lemmy.world:

And then do some more browsing around Lemmy.world, just like you'd do with your phone.
Next, write some other instance's address on your browser's URL field. I'm using Sopuli, so that's one option: Try writing sopuli.xyz as the address. You can try logging in again, bit it won't work, because you don't have a user account on sopuli.xyz.
So, go back to Sopuli's main page and click "Local". You'll see only content hosted on sopuli.xyz. Having browsed those enough, click "Communities" on the upper part of the screen, here:

...and then make sure you have "Local" chosen. Choose any community you reasonably like and click on its name. On the upper right corner, press Subscribe, here:

Clicking that button gives you a dialogue asking "Enter the instance you would like to follow this community from". Write the address of your own instance there, in your case lemmy.world. Sopuli will then tell .world that you'd like to subscribe to that community, and lemmy.world will handle the rest of the subscribing. So, even though you're browsing a completely different instance, you can still also subscribe to communities there.

But alas... Some instances use a theme that has no "Subscribe" button. Which is weird. Try for example suppo.fi . Still, there's something you can do: If you see anything interesting on any other instance, just copy the address from the URL bar, then go to your own instance (lemmy.world), click the search button here:

...and then paste the URL here and press [Search]:

You could also try feddit.org to see how the biggest German-speaking Lemmy instance looks like when you press "Local". Or lemmy.pt to see something in Portuguese :))

But now, let's get to answering the original question: I do not think Voyager has a feature for viewing the content of one single instance only, unless you create a user account there. And that's a bit cumbersome. What you can do is open the web browser on your phone, then open some instance on its browser and either click the subscribe button if it exists on that instance's theme, or copypaste the URL of the interesting thing to the search of lemmy.world, again using your phone's browser. On lemmy.world you'll need to click the hamburger menu on the upper right corner in order to find the login button:

Of course, if you want to subscribe to things with your own account, you need to be logged in :) Whatever you subscribe to with any browser will then immediately be visible on Voyager as well, as it's just another interface for the same Lemmy.

What you can, however, do on Voyager, is press "Posts" on the lower part of your screen (sometimes you need to press it several times):

And then click "All" here:

That will show you content regardless of what instance it was published on. But, as you remember from having browsed the various instances on your web browser, there's a lot of content you cannot see in that view. But, what you can indeed do is open the interesting instances on your phone's browser (or your computer's browser), browse the communities there, and subscribe to whatever you find interesting. Remember, you'll only need to do this once, because once you've found the communities you find interesting and subscribed to them, they'll be visible on Voyager just fine :) If for whatever reason you want to have user accounts on several communities at once, Voyager does support that. Once you've created a user account on some instance, you can add it to Voyager.

And to close this short textlet:
You can see a short list of the biggest instances here: https://join-lemmy.org/instances .
And a full list of all 597 lemmy instances here: https://lemmy.fediverse.observer/list .