CMD is an interpreter; gives users an interface/language to access features of Windows™. It does NOT give direct access to system memory or CPU registers (a feature of lower level programming language).
To "open and select a file" (in explorer window) you would:
explorer.exe /select, "FILE_PATH"
To just open a explorer window to select a file manually, you would:
cmd.exe /c start "" "FOLDER_PATH"
If "an existing Explorer window" is already open, and you want to just select a file manually, you would Activate the explorer window (that is already open instead of opening another explorer window).
Windows manipulation is a feature of GUI and CMD provides a CLI to Windows™, hence you'd need to write code in a (lower level) language or one that gives you access to such features ie. to activate open windows eg. other interpreters (AutoHotKey/AutoIt/VBS), compilers (C/VBA), or assembly/machine code (which can be interfaced via CMD (forfiles or fc) or related utilities eg. Debug or Certutil)
See:
CMD command in Windows to switch to an already open application
A workaround would be to open your chosen folder via explorer "FOLDER_PATH" for manual selection of file or autoselected via explorer.exe /select, "FILE_PATH" & close the existing explorer window via taskkill /im explorer.exe /fi "windowtitle eq %title%"