Part 1

I think an important thing to note before even addressing how combat units may have been controlled in earlier times, even today the notion of clicking and dragging units around a map is nonsense from a realistic point of view. It's a very effective and fun gameplay abstraction, but not how command-and-control would work even with extensive use of radio and a number of other modern technologies.

They way orders work in modern manouvre-based militaries is through what is commonly called "intention-based leadership". As it says in the lid, it entails leading through intentions. The commanding officer will have an 'intention' or overarching plan to achieve some goal(s), and the execution of that is decentralised to junior officers on the tactical level, who in turn will decentralise the execution of fragments of that plan to even more junior (non-commisioned) officers. At each level, soldiers will make decision based on their available resources, intel, the lay of the land and so on. In modern warfare this sort of decision-decentralisation may go all the way down to the fire team or even individual soldier.

A part of how this works is through doctrine. Doctrine is, essentially, "how we fight". Every soldier and officer in a well-functioning military will have a keen understanding of the respective military's doctrine - at least on the level they themselves operate on. So when decisions are decentralised, it is in great part based on a common understanding of how everyone are thinking and acting.

I mention doctrine because it is important to understand how command-and-control was done thorughout the ages. Even if doctrine may not have been codified and written down (note: Sometimes it was. For example, Maurice's Strategikon may not be quite as pedagogic as my manual on squad tactics, it does give significant insight into how the Eastern Roman military of the time operated!) any organised military throughout the ages will have had some sort of doctrine. That is, a common understanding of "how we fight".

Furthermore, that common understanding would be mirrored in an ability to actually fight according to it. Even if the citizen militias of ancient Greek city-states tended to eschew drill and combat-specific training, everyone would know to fight as a hoplite. Medieval knights would know how to line up for a charge and execute it. Napoleonic soldiers would, as you can imagine, be drilled in all sorts of marching manouvres.

A part of doctrine is also how combat units are organised. In a modern manouvre-based military, the smallest formal combat unit is usually the (rifleman) squad, usually lead by a non-comissioned officer, though that can be separated further into fire teams or even buddy-buddy pairs (yes, that is the term where I'm from!). For medieval knights, groups of knights bachelors would follow a knight banneret (a knight with a banner), as the smallest grouping within a hierarchy of retinues-of-retinues. In a post-Marian Roman legion, the smallest unit was the contoberium (in theory this could be 10 men, but usually it was 6-8), which was a part of the usually 80-men centuria, lead by a centurion, which in turn was a part of a cohort, which in turn was a part of the legion.

Realistically, most of the combat units mentioned above would not act independently or even be capable of doing so. A modern rifleman squad is expected to be capable of taking initiative and acting independently within the intentions of their orders. A knight banneret, however, would be unlikely to lead his men on complex, independent manouvres, but rather follow the banner of someone higher up in the retinue-of-retinues hierarchy. A contoberium would not act independently.

Why? Because of doctrine. The "how do we fight" dictated that they needed to stay in certain formations of a certain size to expect victory. A knight banneret taking his retinue and riding off on his own would not be an effective combat unit under most circumstances. Instead, he stuck to his 'battle'.