I started learning How to build professional frontend applications by learning ReactJS. I didn't follow any structured courses I learned it by reading the documentations from websites like W3Schools. Then I created simple applications by using APIs like OpenWeatherAPI to learn it better. I was 15 years old when I built my first Weather application with React. Nowadays I usually use Vite + React + TypeScript to create the clientside of my applications. Vite works so fast and that improves my working speed. TypeScript is very good for error handling and I can catch potential errors early in the development process.
The languages that I have experience with building backend applications are NodeJS and Go language. The first one that I've learned was NodeJS because it was easy to learn and then I built my first MVC social media website with ExpressJS and EJS, for database I had used MongoDB and I built that when I was 14 years old. But currently I'm working on folderhost that one is my biggest project that I've ever made. I love building Minecraft servers and when I host my servers from my home network by port forwarding, my friends didn't have access to my server configurations. Because of this I wanted to build a server application that runs on your VDS, VPS or PC that has static IP and use case of application was to share your folders with web panel. And now I'm still working on this application folderhost. The first release was built by using NodeJS as server and create-react-app for client side, but I realized that NodeJS didn't have a good performance and I couldn't convert it correctly to executable file because of this users had to install dependencies like NodeJS. It was not good for users who don't know what NodeJS is and how to install it. Then I switched my application's backend language to Go language with go-fiber. Now I'm working on bugs and new features on that application, I'm planning to make it Open Source. I'm open to learning other backend frameworks like Spring Boot, etc.