These are all expressions we can use casually to describe intelligent people. Aprt from boffin, they are all adjectives.
Brainy
She's the brainiest student in the school.
Bright
She's a bright girl.
Clever
My dog is really clever, he can do a lot of tricks.
Smart
My brother's smarter than me.
a Boffin
A British English slang noun for a scientist, engineer, or other person engaged in technical or scientific work.
Saanvi is a computer boffin. I always ask her when I need help with my laptop
Sharp as a tack
You are as sharp as a tack if you are able to think quickly and learn quickly.
He's only a young boy, but he's as sharp as a tack.