Where is Electric ground comes in missiles and rockets using high voltages ?
Ground (separate from earth) is an arbitrary point in a circuit which is chosen to be 0V. All other potentials in the circuit are then referenced to this arbitrarily chosen 0V. Ground does not have to be the lowest voltage in a system.

Earth on the other hand is typically a low impedance, high current conductor stuck literally into the earth.

To answer your question, the ground in a missile could be any point in the circuit that the designers chose to define as 0V. A "floating ground" is usually meant to indicate that the chosen point is not "tied" to earth (the physical conductor buried in the ground).

Always remember voltage is the difference in electric potential between two different points and not an absolute value like current (the flow of a discrete number of electrons).