For example let's say I have an automotive site and I have a 'Make' dropdown with "BMW, Chevy, Chrysler, Dodge, Ford, etc" and if select "Ford" I want another 'Model' dropdown to only display only Ford models like 'Focus, Mustang, F150, etc."
Is this a good use case for Query Args? I would appreciate a link or two on how to use Query Args too.
The post or taxonomy field does not support getting the value from a field as an argument on live click. You need to create a custom field type and add some JavaScript code to do that. https://docs.metabox.io/custom-field-type/
Can I make this work if I use the "Hierarchical [X] Whether the post type is hierarchical." checkbox and make the Model CPT a child of the Make CPT or some combination thereof?
Yes, you can create a CPT make with some parent posts BMW, Chevy, Chrysler, Dodge, Ford and child posts. Then use the field post with the field type select_tree or checkbox_tree. Screenshot https://share.getcloudapp.com/rRuWLe2e
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