How does child theme functions.php work with parent theme functions.php? Is it like CSS? [duplicate]

The comparison of CSS and functions.php files is pretty flawed. PHP doesn’t “cascade”.

To answer your question, both files are loaded, child theme functions.php first then the parent’s.

What happens if you want to have the function in the child theme
override a function in the parent theme and the parent theme is not
using if statements like the codex recommends?

You mean, if the parent function is not wrapped in function_exists conditional? Then you can’t override the function. That is basic PHP. Try to re-declare a function and you will get an error. You can’t get around that. The best you can do is hope for a filter that will let you do what you need, and contact the parent theme developer.

This answer might be of interest.
Or this one

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