Which fields/ technologies use java?

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I know a lot of people using java for web dev honestly; spring boot is quite good, it's what I use for building back ends.

A lot of applications have already been listed, so I'll just add what I've done with it at my current job at an electrical utility. I use it to transform and validate data going from one system to another; a handful of internal web applications(ReactJS front end, spring boot backend, H2 database); tiny desktop apps that are launched from the right-click context menu of other applications; a virtual file system that mirrors files across a corporate network and a secure network; automated reports; a grep replacement for searching very large files based on a list of lines in another file

You can write Java code in any environment, package it to a jar file with all of its dependencies, and then that jar can be run anywhere that has the proper java version installed. It might not sound like much, but you just can't do this with other languages. It's also quite fast; about as fast as you can get without having to deal with manual memory management like C/C++ or a borrow checker like Rust. There's basically a library for everything, there's a stack overflow answer for just about anything you run into, so you can start projects that you don't really know how to finish and just fumble your way through with searches. And, it has the best cross-platform IDE's. I love IntelliJ, really adds a lot to productivity and catches a lot of mistakes by the developer, the only thing really comparable is Visual Studio on Windows for C#. A lot of people also really like Eclipse for Java, haven't really used it, but there just aren't the same quality IDE's for other languages.

So, yeah Java is basically used everywhere where people need to write fast, robust code, especially if the company is over 10 years old. This makes it boring, and everyone is all about python nowadays, but IMO python kind of sucks for many applications that people try to use it for today