Here's the science since you asked.
First, yes they turn all the time we don't have disconnecting hubs like 1980s trucks, that's why it's possible to shift-on-the-fly and for auto-4wd to work.
Second yes there is a hard limit, the CV stub axle sits in a "cup"... well, stick a spoon in a coffee cup and try to move it as far out of alignment as you can while keeping the tip of the spoon touching the bottom of the cup. You hit a limit right? This is no different from that.
Third... let's take a closer look at angles that are less than the hard "cup" limit and why they still are undesirable.
Here is an animation of a CV joint in action at angle:
In terms of wear and heat notice that the balls have to travel all the way back and forth between the inner edge and the outer edge as it moves. This is friction and it's constant - sure, it's lubricated but lubricated doesn't mean zero resistance. All resistance (friction, stiction, fighting the viscosity of the grease, the inertial deflection of the balls.. all of it) turns into heat because heat is pretty much the most basic form of energy and we all know energy can be neither created nor destroyed so... when something experiences any kind of friction that resolves itself as heat. That degrades the grease which increases friction and increases heat and expands the metal since metal expands when it gets hot.
This expanding metal increases the tolerances, which puts more of the forces on the "tips" of the grooves in which the CV joint's balls are moving, which makes the joint weaker.
Notice also that when the joint is deflected the arc that the balls travel in puts them closer to the center axis of the axle, so they are up against greater leverage. Think about trying to turn a tough screw with a tiny handle screwdriver vs. a large handle screwdriver - you have to grip it a whole lot tighter if the handle of the screwdriver is smaller right? That's exactly what's going on when the balls in a CV are at less than their center apex.
Notice also how the balls are near the outer edge of the CV cup, where it is unsupported and only on one side of the cup, so there is no incentive for it to stay perfectly round under load.
Hopefully you can see from this that CV's become progressively weaker as they are asked to handle greater angles so if you're going to ask a lot from your truck you'll spend a lot less money replacing parts if you keep your geometry as clean as possible.
