And flakes are just an agreed upon set of outputs (i.e. packages.${system} or nixosConfigurations), and a way to explicitly say where you are importing your nix channel from, as well as defining a way of importing things slightly nicer than fetch functions while still making sure everything is locked appropriately and still updatable either atomically or all at once.
They have inputs and outputs, (outputs is a function that recieves the things in inputs and a self variable and returns outputs of the agreed upon format) and a description.
The format is what allows the nicer command line syntax, because the cli has targets it can now look for.
The inputs are what allows for the lockfile, as now it knows what to put in the lockfile
The fetch functions are already locked because you need to put the hash. So those are locked by default, and not being in inputs is not an issue. anything fetched from the channel is locked to the version of the channel in the lockfile because thats the version of the channel you imported.
This makes importing from multiple channels very easy, and makes synchronizing your channels between machines, as well as between nixos and home manager, much easier, as now they are in the lock file and do not need to be configured via external commands.