protocol – Curl json-rpc to network to learn?

I'm trying to understand bitcoin CPP Peer-to-peer protocol to implement SPV on a Web application that needs to receive encrypted payments.

The problem is that in the examples I've found, the examples are for RPC queries addressed to a complete node that I'm supposed to have, but I want to query other nodes (nodes for which I do not have a node). do not have user: password).

As far as I know, bitcoinj and full portfolios connect to the network, send them requests without a user: well, but I have not found out how these queries are built, or how to test them and try them out. curl help to learn more about how it works.

Is there any API documentation I can consult and a list of nodes I can query?

In addition, I understand that to establish a connection, I should send the version request, but curl I do not have a "version".

I found this documentation on the protocol, but I found no way to test it in simple terms (like curl), to find out what is sent-received.

I want to understand the queries I can perform using this protocol without running a complete node.