Not linear, never “finished” – it’s a cyclical process.
Research throws up more questions than it answers.
Not all questions can be answered – not enough time or resource.
Further research of something leads to refinement.
Can call previous research/concepts into doubt.
Einstein said that nuclear power would be impossible as requires atom to be split at will, we now know that to be possible.
When something new is discovered/invented, new field of research opens up. e.g. when web was invented, research started on how to use it and how to build better websites.