For Steam builds:
Windows: %APPDATA%/Surviving Mars/{64BitSteamID}
Linux: $XDG_DATA_HOME/Surviving Mars/{64BitSteamID}
Mac: ~/Library/Application Support/ Surviving Mars/{64BitSteamID}
For GOG/Paradox builds:
Windows: %APPDATA %/Surviving Mars/Save
Linux: $XDG_DATA_HOME/Surviving Mars/Save
Mac: ~/Library/Application Support/ Surviving Mars/Save
For the Windows 10 store/Gamepass edition:
%APPDATA %/Roaming/Surviving Mars/{MicrosoftUserName}/