Is it necessary to make up (the days) and offer expiation for someone who intentionally broke their fast in Ramadan due to masturbation, and was not praying at the time? And will their fasting of supererogatory days be accepted after repentance?

We hope Allah accepts your repentance, your fasting, and your supererogatory prayers. The majority of scholars hold that you must make up for missed prayers and fasting. The most cautious approach is to make up for what you are certain of, thereby absolving yourself. Regarding masturbation during Ramadan (definition) fasting, it does not invalidate the fast unless ejaculation occurs, in which case only making up the fast is required, without expiation. If ejaculation does not occur, then neither making up the fast nor expiation is required. One should strive to be mindful of Allah and obey Him, and keep away from disobedience.