Yes, 'The Kissing Booth' was originally a Wattpad fanfiction. It gained a lot of popularity on the platform before being adapted into other forms like a movie.
Sorry, I have no idea which fanfiction The Kissing Booth is based on. It could have originated from the creator's imagination rather than being adapted from someone else's fanfiction.
In my 'The Kissing Booth' fanfiction, Noah Flynn decided to surprise Elle at school. He knew she had a big test coming up and was stressed. Noah sneaked into her classroom after school hours. He set up a little study nook with all her favorite snacks and study materials. When Elle walked in, she was shocked but overjoyed. Noah then told her that he would help her study all night if she needed it. They ended up spending the evening not just studying but also sharing sweet moments and memories from their relationship.
Yes, it is. Fanfiction often involves taking existing concepts or characters and creating new stories. 'The Kissing Booth' has elements that can be seen as fan - like in nature, such as the common teen romance tropes and the somewhat idealized high school setting that are often seen in fan - created works.
The Kissing Booth is based on a fanfic. It's not a completely original concept. The fanfic's popularity might have led to its transformation into a more mainstream form of entertainment.