I searched Lemmy in the app store. I didn’t (and still don’t) understand why there are a bunch of third-party apps that connect to Lemmy. But it seemed like Voyager was popular, so I went with that.
Lemmy is not one centralized entity or corporation, so it is impossible for a first party app to exist. Think of it like a collection of separately run clubs/groups that all communicate with each other over the same medium. But the medium is not run by a single person or company.
However, Jerboa is the app made by Lemmy devs themselves, so I suppose it's as close to "first party" as you can get.
There were many third party Reddit apps before Reddit had their official app. And you could argue that Jerboa is the official Lemmy app since it is made by one of the main Lemmy devs.