In this article, I'll explain the process of adding a custom field in a Magento 2 contact form. For the purpose of understanding, I will add a Subject field to the contact form, however, you can add your own fields
Basically module use for implement new functionality and extend the functionality of others module. It does not impact the functionality of others module. Modules will be in app/code directory of a Magento application and format will be: app/code//.