To be able to interact with an ETH RPC, you can send a request via CURL/POSTMAN directly to a contract

For eg, contract - https://etherscan.io/address/0xdac17f958d2ee523a2206206994597c13d831ec7#code

Total Balance

Calling read only function totalBalance

curl \\
  -X POST \\
  -H "Content-Type: application/json" \\
  --data '{
    "jsonrpc": "2.0",
    "method": "eth_call",
    "params": [
      {
        "to": "0xdac17f958d2ee523a2206206994597c13d831ec7",
        "data": "0x18160ddd"
      },
      "latest"
    ],
    "id": 1
  }' \\
  <YOUR_RPC_ENDPOINT>

data is the first 4 bytes of the keccak-256 hash of the function name (totalSupply)

Ref - https://emn178.github.io/online-tools/keccak_256.html

Balance of

balanceOf requires an argument (the address of the user whose balance you want). To find data there, you need to

1. Get first 4 bytes of hash of balanceOf(address)
2. Append the arguments after that (for eg. 0x587EFaEe4f308aB2795ca35A27Dff8c1dfAF9e3f). You need to first convert this to 32 bytes before appending (0x587EFaEe4f308aB2795ca35A27Dff8c1dfAF9e3f ⇒ 000000000000000000000000587EFaEe4f308aB2795ca35A27Dff8c1dfAF9e3f)
3. Final data = 0x70a08231000000000000000000000000587EFaEe4f308aB2795ca35A27Dff8c1dfAF9e3f

curl \\
  -X POST \\
  -H "Content-Type: application/json" \\
  --data '{
    "jsonrpc": "2.0",
    "method": "eth_call",
    "params": [
      {
        "to": "0xdac17f958d2ee523a2206206994597c13d831ec7",
        "data": "0x70a08231000000000000000000000000587EFaEe4f308aB2795ca35A27Dff8c1dfAF9e3f"
      },
      "latest"
    ],
    "id": 1
  }' \\
  <YOUR_RPC_ENDPOINT>