How to customize default add content page I want to customize the default add content page, more specifically, to get rid vertical tab in the bottom, and add explanation text to each field.

How can i do this?
what php file should i tinker ?
is there already some modules to to this ?

thank you for reading this.
You can use the Renderable Elements module. If you've ever had any experience with Display Suite, it works exactly like that, but for forms like Add Content. You can change up the display, remove the vertical tabs below, etc.

For the explanation, each field comes with a description/help field that you can fill out. Now it depends on the admin theme you're using on how that text is displayed.