How to display data every 10 seconds?

Yes it does haver some explanation.
But is does not include an example with real and easy to calculate numbers.

My opinion is: without real numbers it is much harder to understand as with numbers. Anybody who has understoond how it works has no problem to follow the explanation wihout numbers. To anybody who has never heard before about non-blocking timing it is much harder to understand.

So leaving out the real-number-example is what I call
the experts blindness for beginner difficulties.

A teaching-expert is a person that is a double-expert:
expert for the thing itself
and expert for beginners difficulties
best regards Stefan

If you want a framerate control (do a certain amount of thing every X time apart), do this:

unsigned long lastFrameStart;
unsigned long nextFrameStart;
bool justRendered;
uint8_t lastFrameDurationMs;
uint16_t frameCount;
uint16_t frameDiff;


bool setFrameDiff
(uint16_t rate)
{
  frameDiff = rate;
  return 0;
}
bool nextFrame
(void)
{
  unsigned long now = millis();
  bool tooSoonForNextFrame = now < nextFrameStart;

  if (justRendered) {
    lastFrameDurationMs = now - lastFrameStart;
    justRendered = false;
    return false;
  }

else if (tooSoonForNextFrame) {
    if ((uint8_t)(nextFrameStart - now) > 1)
      sleep(1);//delay(1) when the processor does not support "sleep" function
    return false;
  }

  // pre-render
  justRendered = true;
  lastFrameStart = now;
  nextFrameStart = now + frameDiff;
  frameCount++;
  //if (frameCount > 20000)
    //frameCount = frameCount - 20000;
  return true;

The code is part of my library that display graphics on a screen at 50Hz. or 10Hz.

I then used "frameCount" to poll data from a temperature sensor (and/or PM sensor) every 25/5 frames (every half a second) to make sure I don't miss out anything, since the sensors will respond with "NACK" when the data is not ready.

In my experience, I found their explanation good enough for most people.

It seems obvious that the microwave oven count down timer is the same as calculating the difference in the time now and the time at the start.

Of course every learner is different, so some might require this to be explained in more details.

A good teacher would adapt to the situation.

Also, Blink Without Delay is a multitasking demo, with only one task. That limits the conveyance of understanding.

Yes, when I was learning how to use millis() and I looked at Blink Without Delay, my first thought was "Why bother?". Something called Blink 2 LEDs at Different Rates would be far more useful at showing why one should bother after all.