bootserver is a tool that takes a set of images and paves a device. The go bootserver is meant to deprecate the C version at //tools/bootserver_old and will be backwards compatible with it.
The bootserver executable is not actually used by the infrastructure, but the library is used by botanist (which is used by the infrastructure) to pave devices. The executable is built and uploaded through artifactory to GCS along with other build artifacts to allow developers/testers to download it and pave their local devices with the images produced by a particular build.
The bootserver tool can take in specific images as command line arguments to pave either zedboot or fuchsia. It uses the same arguments as bootserver_old. See the flags at cmd/main.go to see which flags correspond to which images and also which unsupported bootserver_old flags are remaining.
Alternatively, you can pave using the -images flag with the images.json manifest produced by a build. The manifest should follow the schema at //tools/build/images.go and include all the necessary images for paving. This will automatically be generated after a gn gen or fx set command in the build out directory, but the images will actually have to be built with ninja or fx build before you can call bootserver with it.
The -images flag takes either a path to a local image manifest on the filesystem, or it can also take a GCS path to an image manifest for a particular build (i.e. gs://fuchsia-artifacts/builds/<build id>/images/images.json). If using a GCS path, the images will be downloaded from the same directory in GCS as the manifest.
The -images flag must be used in conjunction with the -mode flag. The way bootserver determines which files to pave the device with is by looking at the bootserver_pave/bootserver_pave_zedboot/bootserver_netboot fields of each image entry in the manifest, and depending on the mode provided (either pave, pave-zedboot, or netboot), it chooses the images with non-empty args for that mode to send to the device.