"So the starches and sugars in beer and vegeatbles will not thicken but those in oj will.. I need much more practice.
The starches in wine/beer have been converted to alchohol by yeast, any reduction you do will either NOT thicken, or have to be reduced to a very small amount to "thicken".
There are thickening agents, like flour, potato starch, gelatin from bones, etc, which "thicken" due to their properties.
Recuctions are made simply by reducing water content, leaving behind things which aren't water. (to oversimplify things a little bit), if there is nothing left after evaporation, then there is no "thickening". Tomato sauce, for example... thickens because you evaporate all the water content in the tomatoes.
Sugar syrup thickens, because you're mixing water and sugar... then evaporating the water content, thereby increasing the sugar to water ratio beyond what you started with. (this is why juices get thicker as they reduce)