They base their broadcast around hourly cycles put onto this day’s frame, let’s say it’s a 12h circle like on analogue clocks. Each cycle starts with a jingle and ad, so you can divide it into equal 12 parts and eat a 5-minute slice from each. Then you have standardized blocks to peg into them, e.g. 2-3-minutes songs, ads, news blocks, longer blocks to occupy the whole hour like Car talks with Martok etc. Some of them (news, ads, jingles) repeat every hour or two making it trivial. There’s not many ways to slice this pizza, so many of their plans naturally overlap. Iirc on music stations it’s performed-semi automatically now, the machine can compile a plan to fit pieces from your music library and other components by itself.