How does the 4017 decade counter drive a set of servos?
All servos require a repeating pulse to work

depending on the servo, they require a 1 to 2ms pulse repeat about 50 times a second.

What is done is this...

a reset pulse sets the counter IC back to 0
Output 0 on the counter goes high,

A timing loop is run for the first servo (on output 0),
e.g. 1 ms,

then a pulse is applied to the clock input, this sets the counter IC to 1
Output 0 goes low, and output 1 goes high

A timing loop is run for the second servo (on output 1)
e.g. 1.5ms

then a pulse is applied to the clock input, this sets the counter IC to 2
....

I first came across the method of using a counter IC in "Theory and practice of model radio control" - second edition... (c) around 1975

A reset pulse, then variable length sequential pulses applied to the clock pin to both control the servo and select the next servo.