The prebuilt/versions directory mirrors the directory structure found in the Zircon prebuilt Google Storage bucket at gs://fuchsia-build/zircon/. The files indicate which versions to download. The filename to use and where to install the prebuilts is left up to the tool that does the downloading.
On Google Storage, this file exists:
fuchsia-build/zircon/toolchain/aarch64-elf/Linux-x86_64/d1b546ffcd826482cea63ae67a13d3c98a92bf1e
In prebuilt/versions, the corresponding file exists:
prebuilt/versions/toolchain/aarch64-elf/Linux-x86_64/version.sha # the file contains the sha of the prebuilt to download: $ cat prebuilt/versions/toolchain/aarch64-elf/Linux-x86_64/version.sha d1b546ffcd826482cea63ae67a13d3c98a92bf1e