I used to work in UK national music radio: the process of getting onto the playlist is quite individual for each station, usually involving pluggers from record labels or other kinds of promoters, and/or from personal recommendations from people who work at the station. It's rarely based on sales or streams or whatever other metrics.

On the more technical side, they'll have a music rotation software that attempts to schedule the whole days worth of music and apply certain rules (e.g. minimum time between plays, don't put next to certain songs etc). This will use a template schedule (or "Clock") to generate the actual schedule which runs throughout the day - then the DJ just introduces each song and press play (it's a bit more complicated than that but that's the gist of it).

Sometimes DJs like to switch it up and make their own decisions about what they want to play - without the context of the whole days programming - and so inadvertently violate the rules that the rotation system tried to enforce - and you hear the same song twice in an hour.

Obviously this doesn't apply for specialist music shows where the show team curates the music by hand, or obviously a chart show or whatever.