How are transaction fees calculated

[ch1bo]

I'd like to know how the fee for a transaction is calculated currently. I know it did change throughout the various eras of transaction formats. I know it must be in the ledger specs somewhere, but for the latest era (conway) the specification is marked as "WIP" and located in a different repository. Obviously, the fee calculation must be used in many different implementations for transaction building SDKs, tools and the cardano-node itself.

In summary, I'm lacking a simple, understandable description on how the transaction fee is to be computed for a Cardano transaction.