/usr/bin/xbuild is in mono-xbuild 3.2.8+dfsg-4ubuntu1.1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 | #!/bin/sh
exec /usr/bin/mono $MONO_OPTIONS /usr/lib/mono/xbuild/12.0/bin/xbuild.exe "$@"
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