Questions about the JDK, SDK, and Project Language Level

Good afternoon I had Project SDK 10 and language Level 7. Recently I encountered the following problem: In view of this, I began to read and understand the versions of everything that I indicated in the title of the post. As a result, I installed version JDK 11, then solved the problem by setting Project Structure -> Project Settings -> Modules -> Language level to 11. But I still didn’t fully understand what was what. I will be very grateful if you can tell me the answers to the following questions:

1. Latest version of JDK is 11?
2. What do Project SDK and Project Language level define?
3. Should Project SDK and Poject Language Level match each other?
4. How do you know when to change Project Language Level and Modules -> Language Level?

Thank you very much in advance!