The Fediverse is not an app in itself, but rather a collective term for all the services that integrate themselves with the ActivityPub protocol for federation.

This just means all the apps that talk to each other in a specific way.

Now, I don't know anything about Chromebooks or Fire Tablets, but to use Android as an example; I use the app 'Interstellar' to use Mbin. Mbin uses ActivityPub, and therefore is part of the fediverse. Also, I use Moshidon to login to my Iceshrimp.NET account. Iceshrimp also uses ActivityPub and therefore is also a Fediverse service even though it is completely different to Mbin, and I can't log in to Iceshrimp with Interstellar and vice versa.

If you want to find apps for the Fediverse you should look at the websites and source code repositories' readme's to find information about clients. E.G. For Mastodon clients you can check https://joinmastodon.org/apps

Also, if there are no apps for your platform, almost all services let you just use the browser. Like if you have a Mastodon account on https://defcon.social/ you can just open that link in Firefox. Good luck :)