Washing

Food processing plants don't just rinse the fruits in a little water like you or I would juicing an orange at home; they need to wash them with detergent and scrubbers to prevent any contamination. Of course, it goes without saying that this is only possible with the thick insulating peels of citrus fruits.
Extraction

In order to concentrate the juice, they have to get it out of the fruit first. There all sorts of machines and patents dedicated to maximizing the yield from every single piece of fruit. Of course in some cases they also have to de-seed, or preserve the pulp, or both.
Blending

Accounts for seasonal changes in the appearance and flavour of seasonal fruits. Many if not most plants will retain and freeze some amount of juice or concentrate from earlier seasons and mix it with late-season or off-season juices to get more consistent output.
De-oiling

Yes, citrus peels have oil. From what I understand, removing it involves either a centrifuge or a hermetic separator (also common in dairy products).

De-aeration

Removes oxygen (air), which improves both the total output and the shelf life. Notice how homemade orange juice is often kind of frothy and bubbly but store-bought juice isn't? That's why.

Pasteurization

Is a whole area of study in and of itself, and needless to say almost every commercial food needs to be pasteurized before going out the door. This can do all sorts of interesting things to the flavour and nutrition of the juice - some good, some bad, depending entirely on the method of pasteurization.
Debittering

Seems to be done in several different ways, and understand that this is not sweetening. It refers to the actual removal of bitter compounds (such as limonin) via various chemical, physical, and even biological processes.
Acid Reduction

Requires the pulp to be separated first and (sometimes) re-incorporated later, so that the pure juice can be treated to remove some of the citric acid. Again, this is not the addition of a buffer, it is the removal of the acid ions.
Cloud stabilization

I can barely even understand myself, except to say that the opaqueness (or "cloudiness") of orange juice is due to specific kinds of particles and that the process of stabilizing them prevents the juice from turning clear and usually seems to involve pectin (a naturally-occurring gelling agent found in many fruits). I guess consumers would think they were getting ripped off if their orange juice was clear.
Evaporators

Do exactly what the name implies, evaporate water out of the juice, which is where the concentration actually comes from. Again, industrial evaporators are far more complex than the basic principle of simply applying heat, and I'm not going to try to get into that here.
Essence recovery

Deals with the part that's actually evaporated, which is more than just pure water. Processing plants have ways to separate at least some of the characteristic orange aroma from the water vapour and later re-incorporate it back into the concentrate.
Finally the concentrate has to go through freezing and storage and you can rest assured that they don't just throw it in a freezer. The book above makes reference to a "slush freezer" which is similar in principle to the ones you see at the 7-11 but obviously much larger.
Every single one of these steps varies from producer to producer, plant to plant, juice to juice. Sometimes steps can be omitted, sometimes new ones are added. To know exactly what a particular company does with a particular concentrate, you'd have to ask them or visit their plant.