i2p is its own closed off network?

I been trying to understand the difference between i2p to things like Tor and VPNs.

To my understanding, i2p is its own closed off network, meaning you cannot use i2p to access stuff outside of i2p like the clear net, onion domains, etc?

When using i2p, all traffic is relayed through other users in the i2p network like Tor but there is really no such thing as an exit node since you cannot access content outside of i2p?

Therefore i2p is good for hosting .i2p domains that can only be accessed in the i2p network and is good for P2P applications like Bittorrent?

Is this correct? Or am I still misunderstanding i2p?

And therefore could you and other users on i2p run a bitcoin nodes which will only be federated with bitcoin nodes with the i2p network and never federate with the rest of the bitcoin network, essentially making a fork of bitcoin blockchain on i2p?

And is there any other good use cases to i2p besides dark net websites and torrenting?