Ozonolysis mechanism question
The first part of your question has been answered in the comment, as for the part about what does $\ce{Me2S}$ do:
The mechanism used here is reductive ozonolysis and $\ce{Me2S}$ acts a reducing reducing agent itself getting oxidised to dimethysulphoxide $\ce{(Me)2SO}$ (DMSO).
As for the mechanism of reductive ozonolysis with $\ce{Me2S}$ here's a helpful link.