I want to know if I've to rewrite my code for Android

If I make a website with HTML,CSS & JS, I can run this in PC browsers. Would I also be able to run it in Android mobile browsers? Or I have to use different programming language like Java? Also, I want to know is, Android apps like Facebook, Twitter or Instagram, what languages are compulsory to make them?

If you're not using any ESNEXT features and the browser supports the features you're using (https://caniuse.com/), you will be able to use the website on a phone, yes.

If you want it to be a native application (i.e. one you can download from the app store), you could learn a language like Java OR use React Native, which allows you to make mobile apps with similar language syntax to web development.

At the core of every browser, mobile or not, is JavaScript. You can absolutely build a website that runs on them. However, you will have to mind what is actually compatible for those. You can manually check to see if a certain method or object is supported in different browsers by using a tool like https://caniuse.com/ or you could set up an automated solution that would run against your test suite (you could also test with these tools manually, but automation is always preferred) with a tool like https://www.browserstack.com/

As far as your second question, android apps are all written in Java, not to be confused with JavaScript and have to be compiled to run on android. You'll need a compiler like IntelliJ or Eclipse

Edit: as someone else pointed out, you can also use a tool that might compile html/javascript into a native app with a tool like React Native or NativeScript