That’s a genuine JYETech DSO150 powered by an 18650 lithium cell and a boost converter set to 9 V. Make sure you get a genuine DSO150 from an authorized seller, rather than one of the myriad knockoffs; it doesn’t cost much more and tends to reward the right folks.
Anyhow, battery power means you can connect it directly across components to measure what would otherwise be a differential voltage:
That’s the voltage across R1, the 39 Ω LED ballast resistor in the discrete LM3909 circuit running from a 1.5 V supply. Divide the 314 mV peak by 39 Ω to get 8 mA of LED current.
The voltage across C1, the timing and boost capacitor, looks like this:
So the cap adds half a volt to the supply in order to put 2.0 V across the LED, which accounts for the relatively low current; the green LED has a forward drop of about 2.2 V at 20 mA and 1.9 V at µA-level current.
For completeness, the voltage across the LED:
So, yup, the LED really does see 2.0 V. I love it when the numbers work out.
Crank the supply to 3 V and see this across R1:
The LED current is now 1.23 V / 39 Ω = 33 mA.
The capacitor just barely enters reverse charge:
Pop quiz: what voltage to you expect to see across the LED?
I’ll leave further investigation to your imagination, but for low-frequency analog work, you can do worse than a DSO150.