The SCP Foundation is purely fictional. It's a collection of imaginative and often creepy stories and characters that are made up for entertainment purposes and have no basis in reality.
I'm not sure which specific 'the watched netflix true story' you are referring to. There are many true - story based shows on Netflix, and without more context, it's hard to say if it's popular.
Crossover fanfics also attract new fans. If someone is a fan of another franchise and then discovers an interesting SCP Foundation crossover, they might become interested in the SCP Foundation itself. This helps to grow the community. For instance, a 'Star Wars' and SCP crossover could draw in 'Star Wars' fans who are curious about how the two would mix.