For learning the fundamentals, a good source might be geeksforgeeks (https://www.geeksforgeeks.org/multithreading-in-operating-system/) or to look at at a computer science textbook (or even wikipedia). GeeksForGeeks also has a guide on multithreading in Java if that is that language in which you are interviewing.
Leetcode has a few practice questions on concurrency: https://leetcode.com/tag/concurrency/
My other suggestion is to go on reddit and search the subreddit of the programming language you use for multithreading tutorials, there will likely be many posts on the subject.
Discord