trigger jenkins jobs only when a specific branch is updated
Install the git plugin, add credentials for the account you want it to use, then set the job to use SCM Polling. It will check on a preset interval for changes to that repo and trigger when changes are detected. If you need instant triggering upon push, you're looking for webhooks, which is significantly more difficult to set up. I find that polling every 2 minutes or so is good enough in most cases. Here's a decent example: https://www.baeldung.com/jenkins-job-schedule