iOS "Measure" App -- How Does it Work?

I'm using an iPhone 13 Pro, recently upgraded to iOS 16.3, and the "Measure" app came on the phone. I know it requires the lidar scanner that's built into this phone, but I don't understand how it's supposed to be used, i.e., how it operates. Specific questions:

1) What does the weird behavior of the oval around the dot mean. What's it trying to tell me?

2) It appears to measure length in 3D, i.e., even when the two ends are not at the same distance. What are the requirements for an accurate such measurement? Must the phone/camera be held still?

3) One reviewer claimed he could find roof leaks by marking the stain on his indoor ceiling and then going up on the roof and locating the same point. How is this possible, and what's required to make it accurate? Whenever I've tried to find known points on opposite sides of a wall, for example where there's a penetration, I never come close.

In general, a better "user guide" to the iPhone apps would be very helpful! -- JCW6