The keys are inside the repositories section. The repositories are the apt repositories that are needed to complete the build. The keys are the GPG public keys to add to apt’s trusted keyring. Without the public key for the repository apt cannot securely download packages from the repositories listed. This is the same as if you add a new apt source to any computer you need to both add the source url and add the gpg key as trusted. Here’s it in the installation instructions. The only difference is that it’s pulling the key from the key server by fingerprint instead of passing the whole gpg key directly.
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