A19 vs A21 Bulb Size: What's the Difference?

What is the difference between an A19 and an A21 bulb? In short, it's their dimensions.
When wifi smart bulb shopping recently, I got confused between terms like "E26", "A19", and "A21" when trying to pick a size. In this Video, I explain the differences I discovered.
WiFi Smart Bulbs I've Purchased and Reviewed:
A19: Slitinto 1000lm RGBCW - amzn.to/2HLjvZA
A21: Above Lights 810lb RGBW - amzn.to/3c1ZUSU
WiFi Smart Bulb Help:
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A19 vs A21 Size: • A19 vs A21 Bulb Size: ...