What are their roles? and What type of products they design?
SAAS is just an updated version of CSS. You can design front end using SAAS. SAAS is all about front end designing.
A B2B SaaS designer is someone who works on a business product. B2B stands for business to business. SaaS in this case is Software as a Service. It just means an application that isn’t downloaded that you use in the browser. For example, Figma is a SaaS application.
There is nothing super special about it.