Thank you to Paul Maine "The SDR Guy" for submitting his latest video showing how to create a simple spectrum analyzer with zoom capability, using an RTL-SDR and GNU Radio. Paul writes:

Zoom capabilities are discussed in the 3rd edition of Richard G Lyons “Understanding Digital Signal Processing” book. This is a novel approach when compared to other YouTube videos about creating a Simple Spectrum Analyzer with an RTL-SDR.

Additionally, in the video, Paul explains what a spectrum analyzer is and what it's used for, as well as explaining the use of attenuators and discone antennas. In the video, Paul uses an RTL-SDR Blog V4, but has mentioned that an RTL-SDR Blog V3 would work well too.