The Electoral Knowledge Network provides comprehensive information and specialised advice on any aspect of electoral processes (2018).

Wikipedia explains the various electoral systems (2018)  

Vereinigung zur Reform des britischen Wahlsystems (2015)

The world map shows the different voting systems (2018).

Germans elect their members of parliament with two votes (2021).

Alan History Nerd explains how the different electoral systems used in the UK work (UK 2020).

Site of the the body that elects the President and Vice President of the United States every four years (The National Archives and Records Administration 2020)

This information source provides timely and accurate election statistics, electoral laws, research reports, and other useful information regarding the United States electoral system (USA: University of Florida 2018).

Alex Keyssar, Professor of History and Social Policy at Harvard Kennedy School, explores the issue (11 minutes / Harvard Kennedy School 2020).

Short explanation by TED-Ed (2020)