Carrying out multiple synchronous tasks with a micrcontroller
There are several factors involved that make this work well enough for devices requireing less than magic to operate them.

(1) The world passes by slowly when you are a microcontroller: DC motors and servos operating at full cry appear to be standing still to a microcontroller. eg a motor running at 10,000 RPM rotates 10,000/60 ~= 160 revolutions per second. So one revolution takes 1000000/160 =~ 6,000 microseconds. One degree of rotation required 6,000/360 ~= 16 uS. Depending on the uC concerned, 16 uS is starting to get noticeable, but if a processor can handle say 10 MIPS it will excecute about 150 instructions in the time that it takes a 10,000 rpm motor to rotate 1 degree. Generally speaking a uC at this sort of speed can be close enough to being N places at once to a degree that appears instantaneous to most real world devices. 

There are real-world devices that start to make the above assumptions a bit suspect, but in most cases all shall be well.  

(2) True simultaneous operation or close to simultaneous operation is easily enough achieved when needed. 8 or 16 or even 32 digital bits can be written genuinely simultaneously (ignoring possible nanosecond level skew). If a decision is made in advance to implement a set of inter-related actions at a certain time, they canm be implemented essentially simultaneously by sharing a single processor write to port. In practice consecutive writes to ports will occur quite quickly enough for any time differences to be indiscerniblke by mere mechanical devices. 

In the case of PWM signals, these will usually be implemented at a frame rate which is slow compared to processor cycle times. An exception may occur if the PWM uses a PLL clock which is some multiple of the processor clock, but even then a whole frame is liable to be many processor cycles lomg. eg a processor may have a (slow) 200 ns cycle time. A PLL may clock a hardware PWM unit at say 50 MHz or 20 ns/bit. Even then, an 8 bit PWM register will produce a 160 nS frame rate which is of the same order of length s the uC cycle. Few and extremely far between are the hardware devices that need a sub 1 uS PWM frame rate. Anyone attempting to drive a power system at this speed would face horrendous switching loss issues and you would need very special requirements to do this. 

IF some action requires responses which are synchronised to better than a uC cycle time this can be achieved by preloading data and triggering a simultaneous write either to a port or ports at once or by clocking external hardware. It happens, but it's not usually needed in home or consumer level equipment.