To customize the template you would need to use Template Overrides which you can find more information on the wpjobmanager.com documentation site:
https://wpjobmanager.com/document/template-overrides/
If you want to customize any of the form field templates (template for field types), please see this link for documentation on that:
https://plugins.smyl.es/docs-kb/using-custom-template-overrides-for-input-fields/
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For the Packages addon the override directory should be jm_packages instead of job_manager
For Resumes addon the override directory should be wp-job-manager-resumes instead of job_manager