The phrase you choose depends on the situation, how big the favour is, and how grateful you are. If your friend passes you a pencil, you'd say 'Thanks', but if he drives you 100 km, you'd say 'Thank you so much'. It would seem strange to say 'Thank you so much' in the first situation (he'd think you were being sarcastic), and it would seem ungrateful to just say 'Thanks' in the second. In ascending order of gratitude, your sentences are: Thanks. Thank you. Thank you very much. Thank you so much. I hope that helps.

'Thanks' is more casual. Otherwise there's no difference.

"Thanks you so much" and "Thank you very much" are used when you are more grateful and think that a mere "Thanks" is not enough.

There's no difference. It's only that "thank you" is more formal/official, like you would write in an a job application letter. ''Thanks'', on the other hand, is more casual like Jmat has said and it is used more in common conversations between people, friends, workmates, etc.