Most of them are nomadic baggara Arabs from Chad and Niger
Most of the RSF is sourced from Sudanese Baggara; the Sudanese state created that military outfit and it had over around 100k fighters when this war kicked off. The Arab supremacist ideology of the RSF is just another variant of Sudanese Arab racial supremacy.
Chad fought and defeated the precursors of these Baggara Arab supremacists -> Tajammu al-Arabi (Arab Alliance); a Sudanese Arab militia that Gaddafi armed, supplied and unleashed on Nilo-Saharans in Chad; Gaddafi's vision was to create an Arab empire across the Sahel and explicitly told the Baggara that the region belonged to them.
The Sudanese State (starting with Nimeiry) did its part to strengthen the Baggara at the expense of the South; and this support to the Baggara from Khartoum continued with Sadiq Al Mahdi, Bashir and Burhan in Darfur.
Unlike all the naive blacks in this God forsaken continent bemoaning the death of their dear African hero, I'm eminently glad that the Maghrebi dog (Gaddafi) is dead.
Gaddafi was at least useful; Dr. Garang rehashed passages from that so called Green Book of his and that egomaniacal pig was so convinced that Dr. Garang was an acolyte of his worthless ideology, that he gave us mountains of weapons and ammunition with which to fight the Sudanese army.