You don't need to use the official apps on Mastodon, in fact you'll probably have more fun if you use third party apps or your server's website.
You can find out about all the different ways of using Mastodon here:
➡️ https://fedi.tips/which-apps-can-i-use-should-i-use-the-official-app-or-a-third-party-app/
You also don't need to sign up through an app. You can sign up directly on the website of the server you want to join, and use your account on any Mastodon-compatible apps you want. You can even use your account on many different apps at once.