Media Buyers tend to be in their 20's and 30's and mostly women. The Media Planner allocates funds for each market based on the directives from the Media Director. The buyer requests rates from stations and sometimes will tell a station rep what rate is needed to get on the buy based on the station audience delivery.
A buyer knows the demographics desired, the metric used (Cost Per Point, etc) and specifics like ethnicity, gender, etc.
For example, if a buy is against 25-44 women, the buyer will be told to get equal exposure in 25-34 and 35-44
Many if not most agency buys come from agencies directly, although some use buying services to get the best rates and to do the paperwork.
And most buys are not made in the same market as the stations that are bought. Most agencies are in NY, LA, Chicago, Dallas, Atlanta. A few more are in Detroit, San Francisco, Minneapolis. But generally the buyers just look at the delivery and know nothing about the station. Sometimes the supervisors will say "don't buy the same format twice" in a market to avoid cume duplication.
Local agencies can be very different. Often they are small, with one person doing the whole media job. The relationships with local and regional stations can be a major factor. And the client may have "preferred media choices" that have to be considered.