/diy/ - Do It Yourself

>just found out I've been using my bench oscilloscope incorrectly for years

I think I'm retarded. Can someone explain/link me to something that explains how oscilloscopes measure voltages in circuits without a common ground between the scope and the circuit? I figured the measurement would need to be a differential measurement with the scope converting it to a common ground before having the ADC sample it (unless the ADC has a differential input I suppose), but I was looking at these open source scopes:
dpscope.freevar.com/files/DPScope_II_V2_06_Schematic.pdf
https://reference.digilentinc.com/lib/exe/fetch.php?tok=a3dd5f&media=https%3A%2F%2Fs3-us-west-2.amazonaws.com%2Fdigilent%2Fresources%2Finstrumentation%2Fopenscope-mz%2Fopenscope-mz-sch-revg.pdf
And neither seem to use a differential input with an instrumentation amplifier. Wouldn't connecting the scope ground to the circuit's through a probe potentially connect two different potentials together? Also if you have more than one channel wouldn't you only be able to have one ground connection, otherwise wouldn't you potentially connect two different points in the probed circuit to one another through ground?