I'm a backend developer, my father in law has declared me his computer expert
Does Magento place all custom code in the same location?
I'm hoping to determine the location of custom code and any interfaces with added plugins.
In Magento, you will find all the custom code in the app/code directory. So if you wanted to created any custom plugin for your store, you can create the same in app/code directory
You will find this app directory in the root folder of Magento.
To know more about plugins and method to create new plugin, i would suggest you to refer this devdocs link - https://devdocs.magento.com/guides/v2.4/extension-dev-guide/plugins.html
Hope it helps !