Solr Jenkins CI - s390x support

About Apache Solr Jenkins CI. I can see there are various jobs running for Solr https://ci-builds.apache.org/job/Solr/ like https://ci-builds.apache.org/job/Solr/job/Solr-Artifacts-main
Would like to  know how these jobs are setup.  Jenkinfiles are maintained on Github for those jobs or if build/test commands are directly executed from Execute shell option while job configuration? ( I can't find any jenkinsfiles in solr Github repo)
The purpose of asking for this information is that we would like to add one more job which will be executed on s390x nodes/labels.

s390x nodes are already available: https://jenkins-ccos.apache.org/view/all/