Hi, I'm not a expert in modern Arabic history, in a simple manner, it went a little like this;

Hejaz/Hijaz was ruled until the wide Arab rebellion by the Ottomans, who delegated many lower administrative tasks to local power holders (tribal leaders).

The most prominent family and leader to take control after the Ottomans were Hussein bin Ali of the Hashemite dynasty. Hussein became the first king of Hejaz, and he was loosely supported by the British and French governments. Hussein maneged to place his son's on the following thrones of Iraq, Syria and Jordan, briefly the Hashemites held Majority of the thrones on the Arab peninsula including Hejaz (Today the western coast of Saudi Arabia). Hussein bin Ali reigned from 1916-1924.

Following the abdication of Hussein bin Ali (because he declarated himself caliph, he wanted to give up on his secular titles), his son Ali bin Hussein was briefly on the throne from 1924-1925. But the neighboring tribes united under the Imam of Nejd, Abdulaziz bin Abdul Rahman Al Saud or simply Ibn Saud, overthrew Ali and took the kingdom of Hejaz and united it with the redt of the tribal lands. This takeover is also called the Saudi Conquest of Hejaz.

The Saudi family were more astounded followers of the Salafi sub-branch of Sunni Islam, while the Hashemites is accepted as more moderate. The Hashemites still reign in today's Jordan.

References and further reading

Dr. Fattouh Al-Khatrash. The Hijaz-Najd War (1924 – 1925)

Paris, Timothy J. (2003). Britain, the Hashemites and Arab Rule: The Sherifian Solution

Teitelbaum, Joshua (2001). The Rise and Fall of the Hashemite Kingdom of the Hijaz. C. Hurst & Co. Publishers.