I don’t work in pharmacy, I’m not even in the US (I’m in Canada) so you can take the following with a grain of salt.
From what I understand, specialty drugs are indeed a subset of brand name drugs. They could have a generic, or not. Brand name drugs that don’t have a generic equivalent are called single source brand drugs and those that do are multi source brand. So specialty is not another word of single source. It’s really the fact that they are expensive and have low utilization that separates them from other brand name drugs.
The prescription drug lifecycle applies to them as well in my opinion.
As for the relationship, I don’t get what you mean. Biosimilars are another version of a biologic drug created later, usually for a cheaper price, so in a sense, they are a “generic” for the biologic drug. The thing is that biologics are so complex and include living organisms, that you can’t recreate the exact same drug. Generics and Brand name drugs are identical on a molecular level. Biologics and biosimilars treat the same conditions but they are not identical on a molecular level.
I hope that answers your questions, let me know if you’d like to discuss it some more!