On all 3 reel payline games there are 22 physical positions en each reel. Typically they start out with a blank, then it interleaves between a symbol and blank, leaving you with 11 blanks and 11 symbols. In multi-line 3 reel games, there are instances where you may have move than 2 consecutive physical blank spaces. On most 5 reel games, there are no physical blanks between reel positions, in a few games there are.
The slot then maps each physical position into a virtual position. There's a video that joey posted that explains this really well. But basically the more virtual positions assigned to a physical position, the more likely it is to stop there. Also increasing the virtual stop from say 64 stops to say 256 stops allows the machine to have a higher top award because the odds of hitting the top prize go up astronomically.
The symbols do not float around. The program is such that consecutive virtual stops are assigned to each physical stop. The RNG (Random Number Generator) determines which virtual stop each spin will use for each reel, basically 3 numbers for 3 reels. Then the program looks up the physical reel position assigned to that stop and move the stepper motor accordingly so that the correct symbol or blank stops on the payline.