The second theory behind "WhatsApp PXL" is less about code and more about media quality. "PXL" could be a misspelling or shorthand for (as in image resolution).
If you own a Google Pixel (4, 5, 6, 7, 8, or 9 series), you get specific WhatsApp perks not available on Samsung or iPhone: whatsapp pxl
Stick to the official app from the Google Play Store or Apple App Store. If a feature doesn't exist there, it probably shouldn't exist at all. The second theory behind "WhatsApp PXL" is less