You don’t, for the same reason you don’t modify WordPress Core, you extend it or use hooks and filters to modify its behaviour.
So when extending or changing plugins you have 2 options:
Forking
You’ve been creating forks of a plugin so far, but it’s important to change the name when this happens so your new plugin isn’t overwritten. You’ll need to manually port over changes from the original code base
Hooks, Filters etc
Create a new plugin, and use hooks and filters to interact with the plugin. A good plugin will provide filters to modify values, and hooks to allow your own plugin to intercept and adjust things.
You can use this to register new css/js, or disable functionality by removing filters/hooks added by the plugin and replacing them with your own.
You’ll need to read the code of the plugin and refer to its documentation for what’s possible.
Remember, plugin code is just code. Each plugins code shares the same environment so the idea of a parent or child plugin makes no sense. Only the order plugins are loaded and wether they’re activated or not has any bearing here