Each of the countries' delegations send their instructions and wishes for their song (choreography and performance) to the producer of Eurovision. The producer then tries to implement each delegation's wishes, and young local musicians then rehearse the entries with the choreography instructed by the delegations. These stand-in rehearsals are being used to plan and program camera work, lighting and effects, and the recordings are sent back to each of the delegations for review. The delegations can then send back change requests.
When the delegations show up in Stockholm the week before Eurovision, all the technical aspects like camera and lightings are already programmed for the performances, so the delegations and "real" artists can start their rehearsals on stage without spending unnecessary waiting time on directions and technical issues.
It is up to each broadcaster to upload the stand-in rehearsals themselves