mmWave vs. Sub-6GHz 5G iPhones: What's the Difference?

All of the iPhones in Apple's iPhone 12 lineup will have Qualcomm modems that support 5G, but potential buyers need to know that not all 5G networks are equal. There's the super fast mmWave (millimeter wave) 5G and the slower but more widespread Sub-6GHz 5G.
There have been mixed rumors about which ‌iPhone 12‌ models will support which 5G spectrums, but it appears that the fastest 5G technology could be limited to the higher-end ‌iPhone 12‌ Pro models, or even just the 6.7-in