expression
A polite, slightly emphatic expression of gratitude. The どうも adds a sense of 'very much' or 'indeed'.
プレゼントをいただき、どうもありがとうございます。
Thank you very much for the present.
先日はどうもありがとう。
Thank you very much for the other day.
ありがとう is the standard 'thank you'. どうもありがとう is more emphatic, like 'thank you very much'.
どうも alone can be a casual 'thanks', but どうもありがとう is a fuller, more polite expression.
Combination of どうも (very much, indeed) and ありがとう (thank you). ありがとう derives from 有り難い (grateful, rare), literally 'difficult to exist'.