Learn how to make use of tags for your GitLab pipelines, in order to ensure certain jobs run on certain runners.
A cheatsheet for Bitbucket pipelines.
Have Jenkins deploy your auto-scaled application by sending an AWS instance refresh command.
A simple CI/CD pipeline that will get you something simple/basic in place that you can the "build" (pun intended) upon. This way there isn't a steep learning curve.
Set variables for your CI/CD pipelines in GitLab.