Function build_layer 

fn build_layer(
    entries: &[(&str, &Cid)],
    layer: usize,
    blocks: &mut BTreeMap<Cid, Vec<u8>>,
    nodes: &mut BTreeMap<Cid, Node>,
) -> Cid

Builds one node at layer and everything under it.

Every entry handed in has a height at or below layer, and the caller has established that this layer is one somebody asked for. Keys at the layer become entries; runs of lower keys between them become subtrees, each built at exactly one layer down whether or not a key sits there. That last clause is the load-bearing one — see the module documentation.