msg.sender and msg.value

A contract has no users, no sessions and no login. The only thing it knows about who is talking to it is msg.sender.

That single value is the entire basis of identity, ownership and permission in every contract ever written.

The exercise below compiles fine

It just records the wrong thing. Nothing is broken syntactically — the logic is wrong, and only the tests reveal it. This is what most real bugs look like.

msg.value and payable

msg.value is how much ETH came with the call, in wei.

A function can only receive ETH if it is marked payable. Send ETH to a non-payable function and the transaction reverts — a deliberate guard against funds getting stuck in contracts that were never written to handle them.

function donate() public payable {
    donations[msg.sender] += msg.value;
}

The other context values

• block.timestamp — seconds since 1970. Loosely policed, and a validator can nudge it. Never use it as a source of randomness.
• block.number — the current height.
• address(this) — the contract's own address.
• tx.origin — the account that started the whole transaction chain. Almost always the wrong choice. Use msg.sender.