Curl bitcoin protocol for learning?
Nodes communicate to each other using the peer-to-peer protocol, typically using TCP port 8333. They rumor blocks and transactions to each other, request data, tell each other about other IP addresses in the network, ...

Each node implements the level of validation they desire; some download all blocks and validate everything (full nodes), some do that but don't keep the blocks on disk after validation (pruned full nodes), and some download only the transactions they're interested in along with proofs they wee including in the chain without verifying validity.

Independently, some node software also exposes an interface to communicate with users or other software using an RPC protocol. This interface however is not public for good reasons:


It's usually not designed to work in adverserial settings (for example: the RPC interface may expose a command to shut down a node, or dump its private keys, ...).
It's not hardened against denial-of-service (DoS) attacks.
It's not others than the node's operator should be using anyway, as the information provided by RPC is an interpretation of what is going on in the network; it's not verifiable. So if exposed to untrusted party, node software could just lie, possibly defrauding the party using it.