Mail their representative you talked with before or their support and ask them. There may not be a citable source. However, you can write the steps. If you're not able to access neither a citable source nor to specific analysis steps, you can just say this task performed by that company with their proprietary methods.

The best case scenario would be to contact the company and ask them. At least I would do it if I couldn't get concrete information elsewhere (online) i.e. here.