The xdr package contains encoding/decoding of Stellar XDR types.
Most of the code this package is generated.
To download new XDR for code generation:
docker run --platform linux/amd64 -it --rm -v $PWD:/wd -w /wd ruby /bin/bash -c 'bundle install && bundle exec rake xdr:download'
To regenerate the code from the local XDR:
docker run --platform linux/amd64 -it --rm -v $PWD:/wd -w /wd ruby /bin/bash -c 'bundle install && bundle exec rake xdr:generate' && go fmt ./xdr
To download XDR for a different branch of stellar-core, modify Rakefile in the root.