Should I enter into a state of Ihram from Medina, or from the Miqat of the people of Mecca, or from Jeddah, if I intend to perform Umrah and will stop in Medina for one day before heading to Mecca?

As long as you will be traveling from Riyadh to Madinah and then to Makkah, you should enter into a state of Ihram from the Miqat of the people of Madinah (Dhul-Hulayfah / Abyar Ali). This is because whoever passes through Madinah, not being from its inhabitants, intending Makkah in a state of Ihram, must enter into Ihram from its Miqat. The Prophet, peace and blessings be upon him, said: "These [places] are for them and for those who come upon them from other than their people, among those who intend Hajj and Umrah."