What is the ruling on someone who watched pornographic films during the daytime in Ramadan, and ejaculated thereby? And is there repentance for him? And how does he atone for his sin? And is expiation required of him?

What you did invalidates the fast and is a grave sin. You must make up all the days you intentionally broke your fast. Expiation (kaffarah) is not obligatory for you, as it is specific to breaking the fast through sexual intercourse. You must offer sincere repentance (tawbah nasuh) for all sins, and abandon them immediately, seeking help from righteous company, frequently engaging in remembrance (dhikr) and supplication (du'a), and reflecting on the Hereafter and the greatness of Allah.