How can multiple contracts exist in a single file?

Question

Consider the following file

contract A {
}
contract B{
}
contract C{
}

Is a file like that valid? Can a .sol file contain multiple contracts as above? If yes, which contract would the deploy address point to?

Now consider the following file

contract A{
}
contract B{
}
contract C is A, B {
}
contract D is A, B, C {
}

Solidity docs say,
When a contract inherits from other contracts, only a single contract is created on the blockchain, and the code from all the base contracts is compiled into the created contract.
So how does solidity identify which contract to compile the bytecode for, is it the most derived contract? If someone interacts with the deployed address, which contract would be serving them?

Answer 1

You have to choose which one to compile & deploy. Or, you can of course compile everything and only choose the one to deploy. Or, if needed, you can deploy all of them.

The used tooling also influences this. For example command-line utilities such as Hardhat and Truffle always compile everything (unless told otherwise). But for obvious reasons you always choose which contract to deploy.