You may wish to begin by reading the text widget conceptual overview

which gives an overview of all the objects and data types related to the text widget and how they work together.

CSS nodes

textview.view
├── border.top
├── border.left
├── text
│   ╰── [selection]
├── border.right
├── border.bottom
╰── [window.popup]

GtkTextView has a main css node with name textview and style class .view, and subnodes for each of the border windows, and the main text area, with names border and text, respectively. The border nodes each get one of the style classes .left, .right, .top or .bottom.

A node representing the selection will appear below the text node.

If a context menu is opened, the window node will appear as a subnode of the main node.