Is it obligatory to make up and offer expiation for one who ate, thinking that masturbating during the daytime in Ramadan invalidates the fast and does not necessitate abstention for the rest of the day?

Masturbation is forbidden, in Ramadan and otherwise. In Ramadan, its prohibition is more severe, and it spoils the fast by the emission of semen. Whoever spoils their fast by it must refrain from eating and drinking for the rest of the day and make up that day. The Malikis hold that expiation is also obligatory in addition to making up the day. You must repent, abandon this sin, and make up for the fasts you have invalidated.