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In Java Servlets, ServletConfig is a configuration object unique to a single servlet instance, whereas ServletContext is a global object shared across the entire web application. Both interfaces are part of the javax.servlet (or jakarta.servlet) package and help manage initialization parameters without hardcoding them into Java source files.

A Java Servlet is a Java class that extends the capabilities of a server, typically a web server, by allowing it to handle dynamic content and interact with web clients. Servlets are a core component of Java-based web applications and are often used to process requests from web browsers and generate dynamic responses, such as HTML pages.

If they give dynamic response like html page

First, understand that JavaScript and Java completely two different languages. Moreover JavaScript's official name is ECMAScript. So the JavaScript AKA ECMAScript was used basically for frontend web page design in web development. Only for front-end page design animation and control. Though after introduction of NodeJS the ECMAScript/JavaScript is also used for server side.

Now come to Servlet and JSP. These both are progrmmed in Java language in web development from Java's early stage of evolution. Yes there is slight difference between the two. 'Servlet' is a piece of code written in purly Java code and sit on server and handle client-side requests and send output based on the program/code that is handles by another code of Java here like JSP too. So, in JSP (Java Server Pages) programming one can mix Java (core) code and HTML, CSS, JavaScript code (that is client-side browser code output). A JSP code may or may not communicate with servlet to produce the HTML+CSS+JavaScript web page.

In one line:
JavaScript = frontend interactivity, Servlet = backend logic, JSP = backend view (UI generation).

Java Servlets are primarily used to collect user requests from a web browser, process backend business logic, and dynamically generate web content (like HTML, JSON, or XML) to send back as a response. They act as a foundational server-side layer that allows Java applications to communicate seamlessly with web clients over the HTTP protocol.

Java Servlets handle multiple requests concurrently using a single servlet instance paired with a multithreaded architecture managed by the web container (such as Tomcat or Jetty). Instead of creating a new servlet object for every user, the container spawns or assigns a new lightweight Java thread from a managed pool for every incoming HTTP request