Electrical Mathematics ?

"Volt(s)" is an SI unit. Like other SI units, the unit "volt(s)" is often with a prefix symbol that modifies unit magnitude.

For a large quantity of voltage, the prefix 'kilo-" is often used which is defined as "1000 times" of the unit.
Hence, 1000 volts = 1 kilovolt
and 1 volt = (1 × 0.001) × (1000 volts) = 0.001 kilovolt
As 1 volt is really not a large quantity of voltage, it is not necessary to convert it into 0.001 kilovolt.

For a small quantity of voltage, the prefix "milli-" of often used which is defined as "1/1000 time" of the unit.
Hence, 0.001 volts = 1 millivolt
and 1 volt = (1 × 1000) × (1/1000 volt) = 1000 millivolts
As 1 volt is really not a small quantity of voltage, it is not necessary to convert it into 1000 millivolts.

Right.  "milli" is smaller, so you need more of them to make a volt.

volt is smaller than kilovolt, so you need 1000 of them to make 1 kV.

The reverse of 1000 V = 1 kV is 1 V = 0.001 kV.

1000 times is kilo and 1000th time is milli.
1 kV = 1000 volts and 1 volt = 1000 millivolts.

1 kilovolt = 1000 volts.  So 1 volt = 0.001 Kv
It's the same as meters: 1 kilometers = 1000 meters So 1 meter = 0.001 kilometer