Add / Extend
Use the WordPress hooks (filters) which are explained here - Tutorial: Adding a salary field for jobs.
Use a 3rd party plugin such as https://plugins.smyl.es/wp-job-manager-field-editor/ which has a UI for field editing.
Salary and rate can be disable at Appearance > Customize > Job Listings or Resume Listings.
Other functionality can be disabled by deactivating specific addons (plugins).