This is a useful question. Let me reformulate as there is many type of settings -- First you need to activate developer mode in the Settings page:

1. General settings, unrelated to customization, but rather to let Odoo work. You can access them in Settings : Technical / Parameters / System parameters (recorded in the ir.config.parameter database table, or "model" in Odoo language).
   

2. Users, groups and company related configuration like access rights and datas can be seen under Settings : User & Companies.

3. Application parameters that you can set in [Application name] : Configuration page

4. Other customizations that you try under Settings : Technical menus. I guess that these are the ones you want to reset. Some are easy (like emails server), some can creates unwanted effects.
   

So your question is now:

How to find and remove customized parameters added under Settings / Technical, what are the tables that store them ?

You may try a search on your user name in every Technical menu. Use the "Created by" filter.

Please let me know if you find something easier.