A teacher can usually adapt the content depending on its audience, I would not teach the research in my field at the same level to professionals, PhDs, master students, bachelor students, amateurs, or even school students.
If what I'm teaching is fairly complex, it requires a lot of background that I could teach, but I would not have the time to do so, because it would be to the detriment of other students. So, while I usually teach 'from scratch', depending on my audience I will obfuscate some details (that I can answer separately if a question is asked) and usually I will dramatically change the speed of the lessons depending on the previous background, because I need to assume that the student has the prerequisite background to understand at that speed fairly complex material.
As an example, I gave some explanations to a student from zero to transformers, it took several hours with lots of questions, the same presentation to a teacher not in the field took me 1h30 and to a PhD in a related field took 25 minutes, the content was exactly the same, and it was from scratch, but the background in the audience was fairly different.
To give a relevant example, graph theory concepts can be found both in so many real-world systems but also in programming languages and computer systems.