system.xml – How to create a dropdown option as a configuration section to update the options from backend and show it from product details page?

I want to add a select box to the product details page.
Which has several options but this is not a product attribute. It should configure as a global variable.
Admin should able to add new options too from the backend.

I tried the system.xml way and I couldn’t update the new option value for the select box I created.

Can someone help me to do this, please?