Some of the apps are in the app store, some are web based that run in the browser, some are desktop apps (such as Notedeck). For the ones that are not in the app stores, I recommend either visiting the website or github page for each app to find their official download links.

There are a few ways to login to each app.

Most apps allow you to login directly using your private key, but that can also be risky to do too much since you don’t want to compromise your key. Only do this if you have to and if you trust the app.

For browser-based nostr apps, there are signing extensions you can use to sign in. Ably and nos2x are two good ones. To use those, you import your key into the extension, and then when you visit a nostr app in the browser it will detect the extension. The extension then communicates with the app whenever it needs your key, but without exposing your private key to the app.

Android users have a similar solutions for mobile apps called Amber. I’m not aware of a similar solutions or iOS apps yet.