I know ERC20 compliant tokens are interoperable but are Ether and ERC20 compliant tokens are interoperable? If so, where can I find the proof, such as whitepaper, journal, blog, etc.?
Ether is the native value token on ethereum. Its existence is necessitated by the trustless consensus process and it's issued via the mining process. Transfers and accounting are native to the protocol.
ERC20 is a contract interface. They are not exactly interoperable but they present a uniform ABI which is conceptually a lot like an API. ERC20 tokens are issued by and accounted for in contracts at the application layer - on top of the protocol.
A contract would need separate processes to deal with three separate concerns pointed to by this question. First, handling native ether. Second, handling ERC20 tokens that are part of the intended design. Third, (optional but recommended) handling stray tokens from other applications, e.g. unwanted strange species of tokens sent to the contract in error.
Hope it helps.
As I understand 'interoperability' it means that 'systems or programs can interact to each other without modification. Tokens don't interact with tokens. But, for instance, a ERC20-compliant wallet can send ERC20 tokens to another ERC2-compliant wallet because they both support the standard token interface. And this means these wallets are interoperable. But wallets can also send ethers to each other. So the answer is: interoperable are not tokens or ethers but applications or contracts which support exchange of any of them.