Connecting to external blockchain (e.g. Bitcoin or Ethereum)

I am setting a testnet validator node. I followed the instructions and I have a working node. Now I want to create chains.json. At this point, I am a little bit confused. How do I connect my node to, say, BitCoin or Ethereum? Do you have any documentation on how to do this part? Assume I have an Ethereum or BitCoin full node. Now what?