It is easy to set up the latitude system on a planet once you know the orientation of its spin axis since the North and South poles define +90 and -90 in latitude. Longitude is a bit trickier. For the Earth it wasn't defined until the Greenwich Royal Observatory was founded in Great Britain in the 17th century ( I believe). There was no Prime Meridian with a longitude of 0 degrees! This point was set up rather arbitrarily and if it had been set up in New York City, at Harvard University, or at the base of the Sphinx in Egypt, we would have BY DEFINITION and international agreement, a new set of longitude lines for the Earth.
For the planets and other bodies, the latitudes are fixed by the spin axis of the planet, and its orientation with what we define to be the northern ecliptic hemisphere; this is the hemisphere with respect to the Earth's orbit around the Sun, where our North Pole lives. The Prime Meridian is defined by the International Astronomical Union. It is defined on the basis of selecting some prominent landmark at latitude zero degrees on a planet, and DEFINING this as the arbitrary zero point, but it could be more complicated, especially if the planet has no fixed landmarks like Jupiter, Saturn, Uranus and Neptune!!
For Mercury, the Prime Meridian is defined to be the crater Hun Kal which means 'twenty' in ancient Mayan. In fact, this crater sits at a longitude of +20 degrees BY DEFINITION. For Venus, the center of the 'Great Eye' in the atmosphere seen in ultraviolet light is taken as zero longitude. For Mars, the tip of a feature called Fastigium Aryn near Sinus Meridiani is defined as longitude Zero degrees. For Jupiter, there are two longitude systems System I and System II which rotate at two different speeds because at the equator the atmosphere takes 9h 50m 30s to go once around, but at the poles it takes 9h 55m 40s. Features in the Equatorial Belt Zones are referenced to System I, and objects at higher latitudes defined by System II. For the other more distant planets, their latitudes are well defined by their axial tilt which is apparent from the circular bands patterns, but no longitude system has been established since there are no stable atmospheric features to peg their Prime Meridians to!