Neither. It's not a Blockchain at all. It's a protocol with a set of rules - like Email.

We all use email, but we access it through different Apps/Programs.

Nostr is the underlying infrastructure - on top of it are all the Apps (we call these Clients).

This is a a video I found helpful to conceptualize: https://www.youtube.com/watch?v=yIccRIEr2gQ

Built into most of these Clients is the ability to send fractional Bitcoin payments called 'Sats' via a Bitcoin L2 option called Lightning.

Crude analogy would be - it's akin to email's ability to send attachments.

https://nostr-resources.com/ this is good too.