Only insofar as the instance stays online. Pixel Fed is compatible with multiple other platforms such as Mastodon, Missley, Sharkey, and Fire fish. So the global feed on Pixel Fed shows you photo posts from users of all federated instances of those other platforms, too. And they can all see posts from federated Pixelfed instances.
The one you use.
Does it matter which one I use?
Only insofar as the instance stays online. Pixel Fed is compatible with multiple other platforms such as Mastodon, Missley, Sharkey, and Fire fish. So the global feed on Pixel Fed shows you photo posts from users of all federated instances of those other platforms, too. And they can all see posts from federated Pixelfed instances.
It’s the same as with Lemmy, you join a server and you see practically all of them.