I am confused with NFHS rule 7-4-4.
"If a fumble or backward pass is out of bounds behind a goal line, the ball belongs to the team defending that goal and the result is either a touchback or a safety".

I can't imagine a play in which this action would result in a touchback.

Help!?

Team A fumbles the ball at B's 2. The ball goes into the endzone and out of bounds behind B's goal line. It's a touchback.

And Team A throws a backward pass at B's 2 yard line which is muffed into the end zone and out of bounds or is recovered by B in the end zone and the play is blown dead. This too, is a touchback.

REPLY: Or...A10 fumbles the ball at his own 3. It rolls to a stop where B11 muffs the ball into A's endzone where it rolls out of bounds. Official rules that B's muff imparted a new force on the ball. Also a TB --> A, 1-10 at A's 20.

Consider the options.

(1) Is it a touchdown? (ball in possession of A beyond the goal line)

(2) Is it a safety? (Team defending the goal caused it to go into the endzone in their possession or out of the endzone)

(3) If none of the above? A touchback.

2 and goal at the 4. A1 runs Left sweep, and fumbles at the 1. Ball rolls into the end zone, B1 attempts to pickup the ball and knocks it out of bounds.

Safety?

REPLY: The result would be a touchback. A's fumble is the force that puts the ball into B's endzone. B would get the ball 1-10 at their 20.

For a safety, B has to have had possession outside of the endzone and caused it to go into the endzone. If B never had possession, how can it be a safety? These are very basic fundamentals of football rules.

Wrong. The determination is force.
B(R) does not have to have possession in order to apply a new force to the ball.
K kicks to R. R runs to the R10 and fumbles, K picks up the ball and runs to the 5 where K fumbles. The ball comes to rest at the 2 and R in attempting to retrieve the ball accidently kicks it into the EZ and across the endline. Result is a safety. R's force is what put the ball into the EZ.
DO NOT get trapped into this on a kick though. A kick ends when the ball becomes dead by rule, or a player gains possession. This may be what you are thinking of.

Jim is correct. B can "cause" the ball to go into the endzone as well as possess it first. Simply touching the ball that is already moving toward the endzone is not a new force. The concept is "WHO" caused the ball to go into the endzone.