Both are common. https://www.oxfordlearnersdictionaries.com/definition/english/data?q=data data noun BrE /ˈdeɪtə/ ; NAmE /ˈdeɪtə/ ; BrE also /ˈdɑːtə/ ; NAmE also /ˈdætə/ (used as a plural noun in technical English, when the singular is datum)

That depends on where you are in the world. Americans say 'datyta'. I would imagine that most of the resto of the world says 'dahta'.

Personally I usually pronounce the first syllable to rhyme with the English word "day", but sometimes I use the other pronunciation, the vowel which rhymes with "cat". They both sound natural to me, but I don't know whether that's true everywhere (I'm in the US)