SECTION 21 - OUT OF BOUNDS, INBOUNDS, AND INBOUNDS SPOT

ARTICLE 1. PLAYER OR OFFICIAL OUT OF BOUNDS
A player or an Official is Out of Bounds when he touches a boundary line, or when he touches anything that is on or outside a boundary line, except a player, an official, or a pylon.

ARTICLE 2. PLAYER INBOUNDS
A player who has been out of bounds re-establishes himself as an inbounds player when both feet, or any part of his body other than his hands, touch the ground within the boundary lines, provided that no part of his body is touching a boundary line or anything other than a player, an official, or a pylon on or outside a boundary line.

ARTICLE 3. BALL OUT OF BOUNDS
Item 2. Loose Ball. A loose ball that is not in player control is out of bounds when it touches a boundary line or anything that is on or outside such line, including a player, an official, or a pylon.

And

ARTICLE 6. INELIGIBLE RECEIVERS
(d) An eligible receiver who has been out of bounds prior to or during a pass, even if he has re-established himself inbounds with both feet or with any part of his body other than his hands.
Exception: If an eligible receiver is forced out of bounds by a foul by a defender, including illegal contact, defensive holding, or defensive pass interference, provided he attempts to return inbounds immediately, he becomes eligible to legally touch the pass (without prior touching by another eligible receiver or defender) as soon as he re-establishes himself inbounds with both feet or with any part of his body other than his hands. See Article 8, Note 3.

The rules for passing eligibility regarding being out of bounds only apply to offensive players.

Any player - offense or defense - must re-establish themselves in bounds to be considered in bounds. If they don't re-establish themselves and they touch the ball, it's considered out of bounds/incomplete. So if they go out of bounds, take one step on the field and touch a fumbled ball, then that ball is considered out of bounds. But if they go out, take two steps on and touch a fumbled ball, it's a legal recovery.