How do interfaces return values?

Question

I'm looking at the function below and they access the balance of the contract through an interface, but interface functions don't have functionality so how is it returning a value? And the function isn't defined anywhere else in the repo

function withdrawTokens(address token) public {
        require(msg.sender == IOwnable(factory).owner());
        if (token == 0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE) {
            payable(IOwnable(factory).owner()).transfer(address(this).balance);
        } else {
            uint256 balance = IERC20(token).balanceOf(address(this));
            safeTransfer(token, IOwnable(factory).owner(), balance);
        }
    }

Answer 1

Interface is a virtual function that calls the specific function on the specified contract. To make this work the contract being called must have the function external or public, and the function must be defined.

In this case of IERC20(token).balanceOf(address(this)) IERC20 calls the function balanceOf located on the contract token, defined below:

function balanceOf(address account) public view virtual override returns (uint256) { 
    return _balances[account];
}