Here is an attempt to piece some things together about how the King stepped down and the Prince is now in charge
King Salman (Salman bin Abdulaziz Al Saud) was the Governor of Riyadh for 48 years. In that position, he developed connections to every high office in the country. Salman became Defense Minister and deputy Crown Prince in 2011. Then a brother died in 2012, making him the Crown Prince. then another brother (King Abdullah) died Jan 2015 so he became King. His chief personal advisor was his son Mohammed bin Salman (MBS). In June 2017, MBS was appointed Crown Prince following King Salam's decision to remove Muhammad bin Nayef from all positions. Nayef is King Salman's nephew and was first in line to the throne. This move cut with tradition and made MBS heir apparent to the throne. Nayef is now one of the royals whose accounts have been frozen.
MBS was a very bright kid. He began attending cabinet meetings at the age of 12. In 2015, when Salman became King, and he made MBS Defense Minister and Deputy Crown Prince. MBS has his own style and he was somewhat quick to break from some Saudi traditions. People who study Middle East military conflict say MBS' personal style can be seen in the way the Saudis fought in Yemen, Syria, and Iraq.
Saudi politics are deeply rooted in family loyalty, so not everyone agreed with what the Salman family was doing. It somewhat tore the House of Saud in two directions. For example, instead of choosing a family member as foreign minister, Salman chose outsider Adel al-Jubeir (who went to school in Texas), former ambassador to the US. He is only the second person ever not belonging to the House of Saud to hold that office. al-Jubeir is a very bight guy, just like MBS. Also worth noting, he does not hate the Jews and has been somewhat open about it.
Here is some more background. The Saudis had there own fight with Al Queda in 2003. As a result, all the armed forces were revamped and integrated. Previously, the Saudi Arabian National Guard (SANG) existed only to protect the House of Saud from coups. It has its own ministry. The SANG, the army, and the Internal Security Forces now train jointly. This joint command probably lessens any threat of a coup now. So, in hindsight that was a good move that showed a lot of foresight.
Back to MBS -- As Deputy Crown Prince, he made surprise inspection tours of all the ministries, asking to see the books. He then fired basically everyone and replaced them on the spot. MBS replaced them with young technocrats not connected with the family. These guys work 18-hour days. These firings were very popular with the people as it rooted out alot of corruption and inefficiencies.
Then in Sept 2016, MBS and King Salman did something that had never happened in the history of the Middle East. They said "We lied. What we did was wrong." He was talking about the funding Islamic extremists, which went back to the early 1960s as a counter to Nasserism. Good read on this here
https://www.politico.com/magazine/story/2016/09/saudi-arabia-terrorism-funding-214241
Worth noting here that there have been two kings since 9/11. The current leadership did not sponsor terrorism. But their predecessors did.
Just recently, MBS came to visit Trump. The guess is that he told Trump that they were not only of of the business of funding terrorists but also of funding US politicians. Saudi money has been buying American politicians for a long time (Bush Family holla). No doubt Trump was on board with this probably promising 100% support. I imagine him saying "Dilly, Dilly!" You may recall the lavish, over-the-top welcome Trump got when he visited Saudi Arabia.
Saudi-US relations have always been complicated. By the oil industry, by Israel and by the petrodollar. While they owned alot of US pols over the years, we also provided for their defense. So each had something over the other. But that ended when Rumsfeld shifted US personnel to Qatar (2003). That is probably when you can say Saudi Arabia became a full-fledged, independent state, no longer an American protectorate. These days, the Saudis are inviting in the Chinese and Russians. Previous American presidents pressured them to not do that. Trump probably changed that policy ("It's your country").
During thus period, other changes were also being made. For example, King Salman disbanded the religious police and began emancipating women. This part was remarkable in its own right. The Saudis went from "shariah prohibits women from working" to "shariah protects working women" basically in one day. In addition, Salman broke the chain of succession when he made MBS the Crown Prince (soon to be King). MBS is a reformer, if not revolutionary. He reformed the military, and now he is reforming the economy, social norms, and religion. This can be seen in the Gulf Cooperation Council GCC which is trying to move the region off oil. To do this, they need regional and internal stability to attract foreign investment. But, they say MBS wants to do this reform for its own sake, he wants improvement in the region generally.
The biggest impediment to all of this succeeding is the mullahs in Iran.