Here's my attempt to answer these questions:

Nostr is multiple apps and platforms that interoperate together. There are Twitter-like apps, Reddit-like apps and there will be any kind of app you can imagine.

The developer team of each individual Nostr app controls that specific app. You can take control of the content you create but if you choose not to, the data is stored on someone else's devices. Often volunteers who want Nostr to grow and succeed in creating a better internet that is not controlled by a few big corporations. You can be blocked by individual users but you cannot get deplatformed from Nostr. Ever.

Each Nostr app has a way to create a Nostr identity. Once you have created one, you can copy your secret key that was created for you to a login on a different Nostr app. You can modify your profile name of your Nostr identity on any profile page on any Nostr app. Creating a Nostr identity doesn't require you to provide any personal information and you won't be required to do so ever in the future.

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