Jenkins Pipelines: Deploy to Production stage

Hi KodeKloud team,

In the "Deploy to Production’ lecture (under the module: ‘Kubernetes and GitOps’) in the topic “Jenkins Pipelines” , while creating the stage ‘deploy to production’, we had mentioned the when block with ‘branch: main’. When we are running the build, how the pull request for feature-branch/enabling-cicd to main branch of ‘solar-system’ repo is appearing in Gitea? What exactly is triggering this ?