They can have the same function - both can modify an adjective. He's somewhat confused. He's a bit confused/ He's a little bit confused. These sentences have the same meaning. 'Somewhat' is more formal/academic, whereas 'a little bit' ( or just 'a bit') is more conversational.

a little/bit is proportional. you say "you can have a little bit" NOT "you can have somewhat"

"Somewhat" is an adverb that modifies an adjective. He was somewhat faster than the other swimmer. A little, a bit, a little bit can also be used as an adverb to do the same. But they can also be used a nouns as the subject or the object. "A little bit goes a long way." "He has a little bit." Somewhat can't be used in these cases.