Administrative boundaries: Statistics Korea SGIS (KOGL Type 1), modified by vuski/admdongkor (CC BY 4.0). https://sgis.kostat.go.kr https://github.com/vuski/admdongkor https://creativecommons.org/licenses/by/4.0/
Simplified by mapcn-kr: https://github.com/DevMinGeonPark/mapcn-kr
FLASH KOREA projected, styled and annotated these boundaries. River centerlines: Natural Earth (public domain), simplified. https://www.naturalearthdata.com/about/terms-of-use/

The Seoul city map uses shared administrative boundary segments across the Han River to improve the simplified river alignment. The regional map uses Natural Earth centerlines.