In order to ensure that “Eurovision 2021” will take place, all the participants will record their performances from the studios in their countries.
To prevent any problem with the travel restrictions EBU has asked all the participating broadcasters to record the performances of their song from the studios in their countries. These records will be used in case artist won’t be able to travel to Rotterdam in May 2021 due to COVID-19.

This means that whatever scenario EBU will chose, there won’t be any problem with the performers.
How will these video be recorded?
During the recording artists have to perform the song with the stage performance as it is planned for the actual show live.

The live-on-tape recordings should not contain any augmented or virtual reality, overlays, confetti, drone shots, water, use of chromakey or green screen.
For the live-on-tape recordings the Reference Group of the Eurovision Song Contest has approved additional measures to guarantee the fairness of the competition.
Source: EBU