Open Git Lab - We will create Jenkins file in master branch.
Go to master branch and create new file called jenkinfile






Write above content and click on commit changes.
Scan Mutibranch pipeline Now - If you check the logs its found Jenkins file.
If you Go back to my pipeline you will see the first branch is built.
Go inside you see stage view. This is an automatic process.






What if you want to change some stuff in your Jenkins file.
Go to my-pipeline master branch left side bottom you will see the build history click over there.
You will see the option that is replay. It will Allow you to adjust n replay the script.





Make the changes as per above image. Then Run, then click on logs you will see the changes.
You can also restart from stages.