So ... either a somewhat uninformed question (which is totally fine BTW, this is all new for everyone!) or a deep question!

1. To the former ... there are plenty of fediverse apps. They're all mostly specific to a platform such as lemmy or mastodon. Most or nearly all of the lemmy apps are still in beta can still be used if you signup appropriately. I think memmy came out with their official version just recently.
2. To the latter ... yea ... if the fediverse is a thing then why aren't there "fediverse" apps that aren't specific to any one platform but instead allow you to "surf the fediverse" in the same way that the browser lets you surf the web or an email app lets you use multiple email accounts from multiple providers? And ... that's a good question! There should be one IMO! It'd be somewhat tricky to make though and this whole fediverse thing is arguably still a prototype. But, IMO, it's gotten to the point now where this actually makes a lot of sense and it's quite pertinent to pose the question. Arguably, until it's possible to consume and browse the whole fediverse through one single interface, it isn't really a fediverse but something lesser.