Assalamu Alaykum wa Rahmatullah. I have heard that the days in Islam end once Maghrib is prayed… Like Ramadan starts after sunset is prayed– with the moon sighting and all taken into account… If that is the case, then why is the first prayer Fajr and not Isha, and why is Isha the last prayer and not Maghrib. Basically when does the day end and begin? JazakAllahu Khairan!!

All praise be to Allah, and may His Blessings and Peace be on His Last Messenger,
You are right; the day ends with Maghrib. The night of the next day then starts.
When you hear the first, last or middle, you should ask yourself in reference to what?
For instance the ‘Asr is the middle prayer because it is between a day prayer and a night prayer. Dhuhr is the first, for it is the first prayer to be conducted in Islam when Jibreel descended to pray with the Prophet the day next to the night of the Night Journey and Ascent.
The day starts with Fajr and the night starts with Maghrib. The night precedes the day, so the night of the 12th is from the Maghrib that occurs on (right after) the 11th.
Allah knows best.