So I understand that Chief of Staff is an American term, but I believe there generally was an equivalent to the other major powers - Marshall for the US, Alanbrooke for the British, Vasilevsky for the Soviets, and von Brauchitsch / Halder for the Germans. In popular culture, the Chiefs of staff listed above are generally overshadowed by the theaters commanders (e.g. Montgomery, Zhukov, Rommel, Eisenhower), with the possible exception of George Marshall but his fame might stem more from his tenure as Secretary of State and the Marshall plan. So I was just curious about their role and relationship with other commanders
1.) Was the Chief of Staff generally considered the most senior general in the entire chain of command? Or were they seen as equals (or first among equals) of theater commanders?
2.) Were the Chiefs of Staff considered the most competent/important generals in their nation's military?
3.) What exactly was their day to day job? Since it seems that most of the time they would not be directly planning major combat operations.
4.) Were there any unique decisions made by the chiefs of staff that had decisive impacts on the war? (e.g. this is totally fake, but something like George Marshall was a key proponent of the Manhattan Project)
5.) [Germany Specifically] I had trouble pin pointing who the German equivalent of Chief of Staff because Halder is listed as chief of OKH and von Brauchitsch is listed as Chief of the Germany Army. Can someone explain the difference between the two - is Halder in the senior position of the two? Also, is my understanding correct that Hitler assumed the title of Chief of Staff from 1942 onwards?
So, Chiefs of Staff are a really old German concept, which came from the Great Prussian General Staff. The Prussian General Staff was composed of the most senior and qualified generals in the Army, and were used to set doctrine, policy, and national strategy. The Chief of that staff (to which the position you are talking about refers to) was ideally the most senior and qualified general of the most senior and qualified generals in the Army. They would act as the person who sat between the Civilian side of the nation (the King/Emperor, or in the US the President) and the military staff which ran the army.
You are completely right that these generals wernt typically field commanders per say, but rather they were supposed to orchestrate the actions of all the field commanders into one coherent strategy or plan. So, while Eisenhower was responsible for the war in Europe, Marshall was his boss and was responsible for all the various theater commands around the globe.
Now, the specifics of what job the Chiefs of Staff were supposed to fill is really vague, and really depends on the person filling the office, and when they were filling the office. Marshall was a really busy guy, he was responsible for overseeing the planning and grand strategy of the American forces, coordinating that with the Allied Powers, coordinating that plan with his boss FDR, and making sure that the homefront produced enough goods to meet the Army's needs. So Marshall was a pretty high level guy, and didnt really get into operational level planning. Helmuth von Moltke the Elder (and the Younger) on the other hand actively campaigned for the German army and "led" armies into battle. They both planned campaigns, ordered generals around, as well as filling the higher level duties that Marshall did.
Part of this difference came from their command styles, and HvM the Younger is far from an example of a good Chief of Staff. I would argue that his neglect of his duty as Chief was partially responsible for the defeat of the German Army on the Marne. However, those examples show just how varied the job could be. The most important aspect of the Chief of Staff was that they were the highest ranking officer in the Army. They could use that power to cut through red tape and make sure that the machinery behind the curtain operated properly, or they could use that authority to guide a campaign to a successful conclusion. It really all depended on when we're talking about, and what the army needed at the time. And of course this is all radically different from what Chiefs of Staff did during peace time, which was much more managerial.
As for the German Army in World War Two, thats really confusing. I dont pretend to know the intricacies of the relationship between Hitler, the OKH, and the OKW. Both the OKH and the OKW were established as a sort of competing set of organizations which often fought each other for precedence over the German Army. And of course Hitler would often just ignore both of them and issue orders to his field commanders directly. In that sense, Hitler abdicated the responsibility of Chief of Staff and fused it with the political side of the war effort. But just from a general perspective, von Brauchitsch was slowly replaced by Halder in terms of precedence, as Halder was more willing to follow Hitler's lead and rubber stamp his decisions. By 1941, Halder had generally assumed the formal position of Chief of Staff, though he was never recognized as such and Hitler reserved the real power of the office for himself.