I signed up for a couple of Fediverse instances that interested me; Lemmy, Mastodon, and Mbin. The app I use on my phone for Mbin is called “Intersteller”. When I filter by “all” posts on Intersteller, it shows Lemmy posts as well. Are they the same instance? If not, why is it showing Lemmy posts?
Lemmy and Mbin are software. Together with Piefed and Kbin they make up what’s loosely called the threadiverse which is the slang name for different types of software that are similar in nature to Reddit.
Each of those can be installed on numerous servers and become instances. So your instance is Lemmy.ca and runs Lemmy. Mine is piefed.blahaj.zone and runs Piefed. Even though the software is slightly different they can talk to each other. So all Lemmy instances can talk with all Piefed (or Mbin or Kbin) instances.
Interstellar is a threadiverse mobile app. I could also use it to access my account even though my instance runs Piefed and yours Lemmy.
Mastodon is slightly different. It is also software but its nature is closer to microblogging like Twitter or Bluesky. There are also different types of software for this too and each instance of all of those can talk to each other.
It is also possible for threadiverse software instances to talk with microblogging software instances but its still a bit unwieldy and awkward due to differences in how they each present content.