We learn from people who were taught by other people. Those people were taught by other people. Those people were taught by other people.
For example, a professor teaching in a university was once taught by a professor, who was once taught by a professor who …
Even the things we derive on our own is because of what we were taught by others.
Is this loop infinite? Does it start? End?
You don't just learn by being taught, being taught is usually just a faster way of learning because unlikely learning by yourself where you have to be really lucky or make lots of mistakes and be able to improve upon them, you're able to take a shortcut by having someone guide you on that path. It's not a necessity or a requirement to learn it's just usually a faster way of doing it, because you skip the parts that were dead ends.
Though at the end of that journey you still arrive in the wilderness of the unknown where you do have to learn on your own.
I agree with your description. But it is not a loop (circular, end = start), but a chain (not closed).
Leaving the linguistic aspect. The idea is expressed by a metaphor ascribed to Isaac Newton:
If I have seen further, it is by standing on the shoulders of giants.
Fortunately it is an ascending chain, at least in the long term. Generally, this holds not only for the global chain of mankind, but also holds for the local chain within each person.
No, it can't be. Simply because there are other ways to learn than by being taught. Research is precisely the attempt to learn things no one has ever known before. People learn by their experiences, by their mistakes, by happy and unhappy accidents. For all those, it starts another chain of knowledge that hopefully will pass to the following generations. Knowledge can also be lost.
It had to start, since professors are a relatively recent product of evolution. As far as we know, there were no professors among the earliest forms of life on Earth. Fossil records show that primitive professors first appeared in the later Neogene period.
