Chances are they will produce more than one mix. The mix used in a footing is probably a lower strength mix than a mix used in a wall. In a concrete bridge deck, the concrete mix may have slightly different proportions. For special applications, like when you're pumping concrete, that requires a different ratio of components. This all needs to be done in a lab ahead of time It's a very good idea to get that process going promptly, so that the day that you actually need the concrete, all of that paperwork is already in place.
You can use additives to the concrete to affect the performance in several different ways. You can accelerate the rate that the concrete sets and hardens, and that is usually called high early concrete. Typically, that's used when you're placing some pavement and you want to place that in service as early as possible so that you can shift traffic and move into the next phase of construction.
You can add retarding agents, which have the opposite effect, and will slow down that initial setting of the concrete. You may need to do that in the middle of the Summer, with the wind blowing, if you're trying to finish a concrete deck surface. You may find its setting up faster than you can actually control the finishing process. You can slow that process down.
Air entraining, I mentioned briefly, you can control the amount of air by introducing additives. There may be enough air resulting from the natural materials that you're using, but you may also want to introduce an air entraining agent. Water reducing is an important additive. You need to limit the amount of water and use a mix that's relatively stiff. That will produce the highest strength concrete. But there are situations where that could be unworkable. In my experience, typically in a precast concrete yard, they have rebar that's closely spaced and there are post-tensioning conduits, or they could be post-tensioning strand. It's a very dense matrix of material inside the forms, and you need to ensure that the concrete moves around these rebars and fills every possible void. To do that, you may want to introduce a water reducing agent, which is called a plasticizer. That means you can still use small amounts of water, but get a much more fluid plastic mix. That's an excellent solution.
Slump test is a very important tool in controlling the amount of water that you add at the job site. You really can't look at concrete by eye, even after years of doing it. It's a very poor gauge of whether or not there's enough water. There is a video showing that process.