Snapchat ushered in the era of undiscoverable UIs. There were other apps that did that, but Snapchat was the most prominent.
Snapchat introduced the idea that holding down the shutter button in photo capture mode should be capturing a video. Unlike any other camera apps on mobile where video capture is a separate mode readily seen in the UI, Snapchat made it undiscoverable.
Snapchat also pioneered the idea that double tapping the viewfinder should switch cameras at a time when almost all other camera apps had dedicated buttons for doing that. Again that's undiscoverable.
To be fair to Snapchat, their camera interface does also have a button to switch cameras. The double tab is just a shortcut. Other apps that have aped this interface also have a button to switch cameras (e.g. Instagram).
Snapchat introduced the idea that holding down the shutter button in photo capture mode should be capturing a video. Unlike any other camera apps on mobile where video capture is a separate mode readily seen in the UI, Snapchat made it undiscoverable.
Snapchat also pioneered the idea that double tapping the viewfinder should switch cameras at a time when almost all other camera apps had dedicated buttons for doing that. Again that's undiscoverable.