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.
Reacting to SCP fanfiction is complex. There are those fanfictions that manage to build on the SCP foundation in a positive way. They might introduce new relationships between SCP entities or new backstories that are engaging. But then there are the ones that are just not suitable for a general audience. They might contain excessive violence or disturbing themes. It really depends on the individual piece, and you have to be careful when exploring the world of SCP fanfiction.
SCP 106's origin is rather mysterious. It is an extremely dangerous and malevolent entity within the SCP Foundation's lore. Some believe it has existed for a long time, perhaps emerging from some sort of dark dimension or abnormal space.