Urban Design is not 'big architecture'. A friend of mine once described it as design concerned with the spaces between buildings as much as the buildings themselves. I like this definition.
Architecture is concerned with spatial design, that being degree of spatial enclosure - how you adjust that to permit entry of natural daylighting, ventilation and views to the exterior space outside. So you can imagine person A standing in their living room looking out at the spaces that Rem has described, between other buildings.
But at this scale, a 5 metre measuring tape rules - it is all about the distance of that opening from the floor, from the ceiling, from the side wall. A door ope can be as little as 700mm and rarely bigger than 800mm. A one metre wide door is really a monster, yet most people other architects with alot of experience are never, ever conscious to such degrees of rather small adjustments which make a view better, a lighting effect inside a space totally transform it, a shelve at the right height etc.
What i am describing here is pretty much static architecture - just the person walking around their apartment or house or office space. What makes movement in a 10x10x10 metre cube 'dynamic' is the changing modulations of openings, daylight, materials, surfaces, textures, colours and acoustics of surfaces.
Master Planning is when you introduce the element of TIME more into the equation. Here, the best measuring tape of all, is probably your self, and how fast you can expect to walk, cycle, or drive. Because at this scale the person is still only 5ft 8-10inchs on average - but by moving on two feet (every environment is designed for human beings who walk - see how sophisticated robots have so much difficulty navigating simple obstacles in our world) an average human being can cover miles if one had to, or many hundreds of yards quick briskly. That is the 'wonder' about masterplanning - routes, circulation, physical bodily movement.
So things like footbridges, or vehicular bridges with pedestrian parts, things like open spaces, things like monuments, edges, nodes... exist in 4 dimensions - that of space (3D) and time (4D).
However some good institutional buildings like the Stattsgalerie by James Stirling of the Frankfurt Art Gallery by Richard Meier, are designed to be experienced through both time and space, light and shade, views and circulation - just like some narrow winding medieval street in a very old Italian medieval city. So urban analogies can continue into buildings - Rem Koolhaas design a building called the Kunstal which is also worth a look from a point of view of its integration with a park landscape, and a motorway edge and a few other things.
Now with Master Planning also, you are talking about groups of people - apparently 15 people is the most you will get to know in your neighbourhood over a lifetime. Sometimes you will not talk to the people next door to you at all. You are dealing with the provision of spaces for kids to use etc, while being watched by adults. In simple buildings in Architecture, you are just designing for a husband and wife say - whereas in larger institutional buildings you must take into account 'groups'. At the 'groups' size of things, i feel that urban design, urban planning does come into it.
For instance, in architecture i have often designed cultural centres etc. But the people using those cultural centres come from a wide radius around the place, by foot and on public transit routes, or by car routes. So even in architecture sometimes, the idea of a larger community comes into it.
I think their is no easy differentiation between interior design, architecture, master planning, urban design, urban planning. I trained as an architect for about ten years, and still didn't manage to qualify - but i found what was very difficult was doing different project assignments - having to do an interior project and look at interior designers, do a master plan and read Kevin Lynch, design a housing complex and talk to planners about the issues involved, or design cultural centres and have to talk to everyone and consult the 'God of Architects - Which supposedly is a guy from Europe called Koolhaas who makes no sense whatsoever!'.
I suppose what the original thread, was really asking is how can i just narrow down the choice/focus to something that i can manage, and do as a college degree, career and job. Noone can accomplish excellence in all fields - but people like architects do know alot about everything - they just don't know much about anything. Whereas, urban planners, may have experience and understand an awful lot about housing, or transport, or land usage, or road infrastructure - and may even understand the economics and politics part of these projects too. An architect or urban designer i think, has to be good at talking to alot of different people, and co-ordinate the project, but not allow him/herself to become too bottled up with any particular aspect. Not easy, not for me anyhow.