Editing Forms under includes folder

The supported template files for template overrides you can find here:
https://github.com/Automattic/WP-Job-Manager/tree/master/templates

I do not recommend attempting to override any of the class files as you will most certainly have issues.

https://wpjobmanager.com/document/template-overrides/

Maybe it would be better for me to tell you. We want to change the checkout order that the employer can select their package and then pay immediately before posting the job.

If we want to stick to the conventions of WP-Job-Manager what is the best way to do this?

If you use the WooCommerce Paid listings plugin, you can change the flow to select a package, then fill out they listing, then pay…

Or if you want to do it with your own custom code look in the submit form class, there is a filter for “steps” .. You can use this to inject your own custom steps