Planners versus architects (an architects perspective):

When looking closer into the world of design, one will find a distinction between planners and architects. While Architects may act and function as planners, the reverse is generally not true. Urban planners are trained to focus on a larger picture and incorporate varying functions of a place. These functions may include buildings and other structures, but also consider infrastructure and landscape concepts and the use of structures. In short, it is urban planners who try to figure out what happens where. Residential living mixed with industrial production? Large trucks and trains mixed with pedestrians and bicyclists? Maybe not a good mix and match…

https://www.planetizen.com/node/68464

There are many schools of thought about proper planning and most of us are living in the result of a certain theory in time that won over another idea. Compact live/work areas typically do not blend with strip malls and suburban single family home planning. High density downtowns will not blend with agricultural production – even though urban farming is a strong and functioning concept!

Once the general ideas have been laid out, the architect typically takes over to designing the actual buildings and structures. To learn more about planning: Jane Jacobs, smart cities and CNU may are good starting points.