CSS Inline defines the root inline box as

The root inline box is an anonymous inline box which is automatically generated to hold all of the inline-level contents of a block container (if it has any).

CSS Pseudo-elements defines ::first-line as

The ::first-line pseudo-element’s generated box behaves similar to that of an inline-level element, but with certain restrictions.

A user agent must act as if the fictional start tags of a ::first-line pseudo-element were nested just inside the innermost enclosing block-level element.

So ::first-line is a restricted inline-level, and inline-levels are wrapped inside the root inline box, which is inline, and inline-level fragments in the first line are wrapped inside the ::first-line.

Which one is inside of which one?