Hello,

for concept, you can prefer Unacademy. for preparation like appearing  mock test or previous year paper, you can prefer gradeup app.

You can also use apps of digibook technologies. for online test series you can prefer CA test series. There are many other apps which can help you in your preparation. You can use them.

for Course, you have to look online webpage of different course providing organizations. You won't get course in online app. You can get subscription package which will provide you materials only.

Hope it helps!