In my experience, these are just labels and they vary quite a bit.
I have worked as a police officer who focused on crime scene investigations. So I was a police officer but I would stop to handle major crime scenes, as my agency did not have a civilian crime scene technician.
A crime scene technician is often a civilian who responds to crime scenes. A crime scene technician takes photographs, collects evidence, and produces crime scene sketches.
A forensic science technician typically works in a laboratory focusing on a highly scientific field, such as chemistry or biology.
Again, these job titles are not strict!