How is ozone formed in the stratosphere?
The textbook answer is the "Chapman Process"
The Chapman Model of the formation of ozone, devised in the 1920s, conjectures an interaction between oxygen molecules and ultra-violet radiation. Oxygen molecules are split into individual atoms which then interact with oxygen molecules to create ozone.
There are problems with the Chapman Model resulting both from a theoretical approach and from later attempts to test it experimentally in order to compare the model with reality.
Some of these problems were investigated by Dr. Michael Connolly and Dr. Ronan Connolly and others around 2014. There was a revival of interest around that time in finding out what, if anything, causes the concentrations of ozone in the upper atmosphere to vary with time and space, as conjectured by Crutzen, Molina and Rowland and others around 1973.