Can't wage a war without officers, and this is doubly true for war with such chaos, as WWII. On that note, Hearts of Iron IV is getting some changes to the officer corps with the No Step Back expansion.
With the new changes the branch chiefs, theorist, and military high command have found their way to the officer corps screen, though for ease of access you may still view and appoint them in the country overview screen like before. This kind of change is the sort of thing that comes up during playtesting - while it made sense to collect similar things together, there was no good reason to change the player’s flow expectations.
The manner in which you’ll appoint advisors has changed a bit. Paradox have decided during the officer corps development process, to make a bigger deal out of the advisor ‘level’, such as specialist, expert, genius, that all non-theorist advisors possess. In addition to adding a flat command power allocation, that is reduction of max command power, which is reduced by high advisor ranks, political power costs are raised by having a higher rank advisor.
Branch advisors now grant daily experience gain, meaning stacking your command cadre well is vitally important to the pre-war development of your military. To add to the choices, doctrines now cost experience rather than being something you spend a research line on.
The most important part of cultivating a strong officer corps is the ability to give your trusted commanders advisory roles. Commander traits earned in active combat can make your characters eligible for specific advisory roles.