The iPhone 6 and 6 Plus feature 1GB of RAM, whereas iOS 13 and later versions require at least 2GB of RAM to run efficiently. Because new emojis are bundled into system software updates, the iPhone 6 lacks the official path to receive the iOS 13 emoji set.

Getting iOS 13 emojis on an iPhone 6 is challenging because the ; it is officially capped at iOS 12.5.7 . While you cannot "download" these emojis into the native system keyboard normally, you can use workarounds like third-party apps or jailbreak tweaks to bridge the gap. The Limitation: Why iPhone 6 Stopped at iOS 12

The easiest way to get an "iOS 13-like" experience without modifying your system is to use apps from the Apple App Store that provide their own custom emoji libraries. How to update iPhone 6 Plus to iOS 13 - Apple Community