The domain knowledge applies to any product for that domain. The point is not to rediscover existing knowledge, but to reuse the models of knowledge. Of course, there is always the problem of deciding precisely what constitutes a domain. The definition of a domain lies in its interfaces with other domains. It is what enters and leaves that defines the domain, not the domain name.
Domain analysis is a long-term project.
That is, the knowledge gained is reusable, but this is of benefit only if you get the opportunity to reuse it. In the case of domain analysis, the investment is rewarded if the domain knowledge is used by several projects in that domain. And keep in mind that there is no limit to the number of times that domain knowledge can be reused. Research and experience have provided many models for defining various aspects of requirements. For example, right at the project start you can make a system visible by drawing a context diagram to model the intended context of the work.