Substrate is a modular blockchain framework underneath Polkadot.
Users are given the flexibility to create and modify their Substrate based chain however they want, which means that there will not really ever be an offline list of APIs available for all chains.
Instead, the Polkadot / Substrate ecosystem relies on Metadata, which you can learn more about here: https://docs.substrate.io/build/application-development/#metadata-system
By querying the metadata of a chain, you can learn about all its APIs and features, and this is the only way to get the data you are looking for in a consistent way.