One of the best tools I know is SoloLearn. In its app description is written:
"SoloLearn has the largest collection of FREE code learning content, from beginner to pro! Choose from thousands of programming topics to learn coding concepts, brush up your programming knowledge, or stay aligned with the latest coding trends."
You can download the app and the courses on Android or iOS or use the application web-based. You can also download the courses in offline mode and therefore, you can continue studying and solve quizzes even when you are not connected to the internet.
