How do you back up Jenkins jobs & master configs?
There are many ways to do this but the easiest way I can think is doing a backup of the Jenkins Home folder.

You can see where is your Jenkins home with:

echo $JENKINS_HOME


And for example, if you only want to backup the jobs you can go to:

cd $JENKINS_HOME/jobs


And make a backup for that folder.

All that configuration will be a bunch of XML files.

If you are using the official Jenkins docker image, the home will be on:

/var/jenkins_home