You seem to be misunderstanding the terms round and turn.
The Order of Combat
[...] A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn.
So a round of combat takes about 6 seconds and within that (roughly 6 second) round, every creature takes their turn. What this means in terms of game-world time is that each creature is essentially acting/reacting all at the same time, within that 6 second time period.
This is, of course, only an approximation. A combat round consisting of 100 creatures might very well represent longer than 6 seconds of game-world time, whereas a combat round consisting of only 2 creatures might very well represent less than 6 seconds of game-world time, even if mechanically, by the rules, a round still takes about 6 seconds regardless of the number of combatants, for the purpose of spell effects/durations and whatnot.
Changing the representation of time in a round of combat based on the number of combatants may not be "by the rules" but there are times when it makes sense (e.g. infinite creatures being able to transport via plants as long as they all take a turn in a combat round).
Even if you're not confusing turn and round; in 'real' time creatures aren't just doing one thing within that 6 seconds either. Even if, on your turn, the only action you take is to attack, that doesn't necessarily mean you spend an entire 6 seconds standing still and attacking in the game world.
Movement and Position
In combat, characters and monsters are in constant motion, often using movement and position to gain the upper hand. (PHB. 190)
Creatures are constantly in motion even if they're not moving out of their space. They're breathing, they're shifting the footing, they're twisting, they're turning. They're doing a whole range of things other than just attacking. How exactly these actions, a creature's turn, or the entire round is narrated in terms of game-world time is up to the player and the DM.