They vary...
Some are from a certain age group, say 18 -25 yr olds, so if it is the top 100 films - you will only ever see recent ones listed, not any classics of 20 yrs ago, due to them not having seen them...
Often it is a select choice of celebrities or comedians for the 100 best comedians or film stars...
The recent best british female actors and best male actors (British) were by half a dozen actors/actresses...
On the songs: best singer: best lyrics: best romantic song: etc etc are usually people in the media i.e. D J's music journalists, singers...