The most important rule: always specify either an absolute or relative file path to whatever you want to launch – except if the file is located in a folder listed in the PATH environment variable:
$env:path
So, to launch the calculator, all you do is …
calc
… since the application is located inside of a "well-known" folder which is listed in the PATH environment variable.
To run a file that is located inside the current directory, you can use "." as a relative path:
cd "$env:programfilesInternet Explorer"
.iexplore.exe
.iexplore.exe