This is the exact crux of the matter, and your confusion is completely understandable. You've identified the brilliant, hidden secret of aircraft instrumentation. The answer is that **the altimeter doesn't measure what you think it measures.**

It's not a radar altimeter calculating geometric height. It's a **barometric pressure gauge** disguised as an altitude readout.

Let's use an analogy:

Imagine you're driving a car and you want to always stay exactly 10 feet above sea level. You have a magic "sea-level-ometer" that just tells you your height above sea level.

*   If you drive from Los Angeles to Denver, your "sea-level-ometer" would start near 0 and climb to over 5,000 feet. You would have to constantly adjust your car's "altitude," which is impossible. This is the "geometric height" model.

Now, instead, you have a different instrument: a **"road follower."** This instrument is programmed with a map of all roads. You tell it "Take me to Denver, but always keep me on the road surface." The road is like the constant-pressure level in the atmosphere.

*   The road itself goes up and down over hills and valleys. To stay on the road, your car must go up and down with it.
*   **But your instrument doesn't show you the up-and-down motion relative to sea level.** It just has one simple display: **"ON ROAD"** or **"OFF ROAD."**
*   Your only job is to make tiny steering adjustments to keep the needle on **"ON ROAD."** If you deviated and started driving in a straight line off a cliff, the needle would immediately swing to **"OFF ROAD."**

**This is exactly how a pilot's altimeter works.**

1.  **The "ROAD" is a specific air pressure.** Let's say 4.36 inches of mercury, the average pressure at 30,000 feet.
2.  **The altimeter is the "ON ROAD" gauge.** It is mechanically or digitally calibrated so that when the outside air pressure is 4.36 inHg, it points to "30,000."
3.  **This layer of air (this "road") curves around the Earth.** It is not flat.
4.  **To keep the altimeter reading "30,000"**, the aircraft **must** follow this curved pressure level. It must make the tiny, continuous "nose-down" adjustment to follow the road. If it didn't, it would fly "off the road" (i.e., out of that pressure level) and the altimeter would change.

**So, to answer your question directly:**

The instruments "know" to adjust **because the pilot (or autopilot) is flying to keep the needle locked on 30,000.** The act of successfully keeping the needle locked **IS** the act of making the curvature correction.

*   If the plane didn't adjust, it would fly a straight path.
*   On a straight path, the pressure would drop rapidly.
*   The altimeter, being a pressure gauge, would **instantly show the altitude increasing** to 31,000, 32,000, 35,000 feet.
*   The pilot, seeing this, would quickly push the nose down to get back to 30,000. This nose-down correction is what bends the flight path to match the Earth's curve.

**In a fighter jet at 10,000 feet, the principle is identical.** The pilot commands a pressure altitude and then *flies the plane* to keep the needle there. The very success of holding that altitude is proof they are curving with the Earth.

The magic isn't in the altimeter passively knowing the curvature. The magic is that the **goal of keeping a constant pressure reading forces the aircraft onto the necessary curved trajectory.** The altimeter is the feedback mechanism that tells the pilot if they are succeeding or failing to follow the curve.

It doesn't register the adjustment visually because the adjustment is the *input*, and the constant altitude reading is the *successful output*. You're seeing the result of a perfect correction, not the process. It's the ultimate example of the instrument hiding the complex physics to give the pilot a simple, actionable piece of information: "You are on the correct path."