The only affected undead are those who fail their save.
Roll a d20 save for each undead
{in your question} The zombies in the area that fail the save are instantly destroyed. The zombies that passed are now turned and will move as far away as possible during their turn.
Not quite.
Channel Divinity: Turn Undead
Each undead that can see or hear you within 30 feet of you must make
a Wisdom saving throw. If the creature fails its saving throw, it is
turned for 1 minute or until it takes any damage.1
If the creature passes its saving throw, it is unaffected.
Destroy Undead.
Starting at 5th level, when an undead fails its saving throw against
your Turn Undead feature, the creature is instantly destroyed if its
challenge rating is at or below a certain threshold, as shown in the
Destroy Undead table.2
\begin{align}
\text{Cleric Level} && \text{Destroys Undead of CR}\\ \hline
5^\text{th} && 1/2 \text{ or lower}\\
8^\text{th} && 1 \text{ or lower}\\
11^\text{th} && 2 \text{ or lower}\\
14^\text{th} && 3 \text{ or lower}\\
17^\text{th} && 4 \text{ or lower}\\
\end{align}
If the creature passes its saving throw, it is unaffected. Undead only are forced to move away, or be destroyed, if they fail their saving throw.
Example:
Four zombies and a wight (CR 3) are approaching the level 5 cleric. The cleric Turns Undead.
All five roll their saving throw (let's call the DC 13 for illustration purposes)
Zombies roll 5, 9, 17, 13, so two are destroyed and two are not affected at all. They keep on coming, moaning "Brains!"
The Wight rolls an 11. It runs away, since its CR is too high to be destroyed and it failed its save. Had the Cleric been level 14, then the Wight (who failed its save) would have been destroyed.
1 Basic Rules, p. 24
2 Basic Rules, p. 25