Is there only one method? for miner and pool communication? I tried doing some sort of filtering on XMRig using wireshark on an ETN pool and I found that XMRig was using login method of JSON-RPC 2.0, my question is does all miners use this method? By "all" I mean even the bitcoin, monero and all minable cryptocurrency miners use this method? I heard about GBT, stratum and getwork, are they just a simple "sub-part" of JSON-RPC?
JSON-RPC as a protocol does not specify other protocols that use it. Rather it just defines the general structure that protocols using JSON-RPC should use. JSON-RPC does not define any methods; login is a method that is offered by protocols using JSON-RPC.

GBT, stratum, and getwork use JSON-RPC. They themselves are not JSON-RPC or some subset of JSON-RPC. Furthermore, none of those have a login method.

There is no guarantee that all coins use the same protocol (GBT, stratum, or getwork) nor that they use JSON-RPC. Typically coins will use one of those or something similar because they simply copy existing code, but there is no guarantee.