The U-tube video. He doesn't understand what's going on. The first mistake he made was "he know" the voltage expected and he didn't select the lower sensitivity first. Bad. It's a good way to blow up a scope.
He never left the probes on long enough, nor did he show the result being AC coupled.
His measurement is a "special case". Why is it a special case? Because he is measuring a low Z source.
You have to now understand a bit about oscilloscope probes. I'm glad I had the lab in school. Look at figure 3 here:
https://www.electro-tech-online.com/custompdfs/2012/07/lecroy_probing_tutorial_appnote016.pdf
The probe is an "impedance divider". It has a series resistor and a capacitor and the scope's input is modeled as a 1 M resistor in parallel with likely 22 pf (Typical). Watch 50 ohm Z scopes. So, compensation means you put the probe in x10 mode and connect it to the scope calibrate pin which spits out a square wave.
You attach your scope probe and it doesn't look anything like a square wave. Why? The Z of the scope's cable + compensation cap and the probe's capacitance are interacting and rounding the corners. What do you do? You adjust the little screwdriver cap until the wave is square.
This is a bit of an aside, but you still need to know it.
So, you x10 probe is now a series resistor in the probe and the 1 meg resistance of the scope. We cancelled the caps by doing the compensation. so, it's behaving as a divide by 10 voltage divider which is called a X10 probe. Who knows where the terminology came from.
What does this mean?
Take a look here: **broken link removed**
and plug in 1,000,000 for the scope R. 100V and 10 V for a divide by 10 and you find R = 900,000 ohms. So, you cable is EFFECTIVELY a 900,000 ohm resistor. There is other stuff going on, but don't concern yourself with that now.
So, what is the measuring circuit then?
He has a nearly ideal voltage source, a voltage source in series with a 0 ohm resistor. This is in series with a 900K resistor and it's in series with a 1 M resistor of the scope. Not much of an error.
Now suppose his voltage source wasn't ideal and was 100k. So there is a 9 V source, a 100K resistor, a 900K resistor and a 1 M resistor. There will be a measurement error.
Remember, that internally, the scope has a 1 M resistor connected to ground. Remember those invisible wires. So, A-B does work without the ground connected. You should get cleaner measurements if you can ground the circuit under test, though.
Your scope probe effectively does have a 10 M input Z in the x10 mode and it does place a 1 meg resistor to ground at the scope's input. The probe tip sees 10 Megs with respect to ground. The scope sees 1M with respect to ground.
So, the A-B mode still has a reference to ground even without the ground connected.
The usefulness of the ground clip comes into play when you are troubleshooting high frequency signals on an isolated circuit.
Now you have something to sleep on.