In Jenkins pipeline, we have two scenarios:

1. Write a Jenkinsfile and check in with the source code. Then while creating job, Jenkins Pipeline will detect the Jenkinsfile and run the job.
2. Create a pipeline in the BlueOcean for the application and mention the steps that you want to perform on the source code. Jenkins pipeline will automatically create a Jenkinsfile and push it to the branch you will give.

Either we write the Jenkinsfile and then run the job or create a pipeline which will generate the Jenkinsfile for you and then run the job.

We will see both the scenarios here. First of all, we will see the second scenario, which will generate Jenkinsfile for you.