Forgetting what you have published
People usually do not ask: "explain this paper" they will read a paper and may ask a more specific question about a part they did not understand.

The professor may or may not be able to answer. If he wants to, he can look up the paper or the answer to the questions asked. His experience and knowledge will probably help him to find the answer faster than the person who asked the question. If he does not want to, or cannot answer the question because he forgot, does not have time, or for any other reason, the person asking is simply out of luck: they will have to ask someone else or do research themselves to find the answers they are looking for.

Nobody can remember all the details about 90 highly specialised scientific papers, especially if they involved lots of work by different authors that may have been done decades ago. Professors and other scientists are human beings: they forget things and make mistakes just like everyone else. This is not a problem.