Can we say "they are somebody who deserve it"? Meaning they are the kind of people who deserve it. If so, should we use third person version and write "deserves"?
Somebody is singular. They (i.e. one person, non-gender, he or she) are somebody/someone who deserves it. They (plural) are people who deserve it.
Deserves is third person singular; deserve is third person plural.
It depends if you're using they as a plural pronoun or as a singular gender-neutral pronoun—something which has recently become a stylistic choice.
Depending on your use of they, there are a few grammatically correct options:
They are (the kind of) people who deserve it.
This is used when you are talking about more than one person.
They are (the kind of) person who deserves it.
They are somebody who deserves it.
This is used when you are talking about a single person of indeterminate gender.
The second examples sound correct to me (as I've become used to using they in a singular sense), however they may not sound correct to everybody. The other stylistic options include he or she is or a rephrasing.