I've seen 2 variations of the RC low pass filter, sometimes with both the resistor and capacitor in series, and more commonly with the capacitor going straight to ground. What is the difference between these variations?
Think of the capacitor as having very low resistance at high frequencies and very high resistance at low frequencies.
So in your top diagram the high frequencies zip through the cap to the output while low frequencies have a harder time. So it’s a high pass. It doesn’t make any difference if you put the cap in front of the resistor.
In the bottom diagram the high frequencies are shunted to ground, leaving only the low frequencies to make it to the output. So it’s a low pass.