There are infact two folders which are always used as well as an optional alternative startup folder if you need it.
These directories can be used to store your global templates.
There is infact a third folder which can also be used as an optional alternative folder.
Any templates (or workbooks) that you save in any of these folder locations will be opened automatically when Excel starts.
If there are any workbooks in any of these folders then the blank default workbook will not be displayed.
xlStart - Machine Folders
The exact locations of the xlstart folders will depend on which version of Excel you are using.
Excel 2016 - C:\Program Files(x86)\Microsoft Office\Office15\XLStart\
Excel 2013 - C:\Program Files(x86)\Microsoft Office\Office15\XLStart\
Excel 2010 - C:\Program Files\Microsoft Office\Office 14\XLStart\
Excel 2007 - C:\Program Files\Microsoft Office\Office 12\XLStart\
xlStart - Personal folders
Excel 2016 - C:\Users\"user name"\AppData\Roaming\Microsoft\Excel\XLStart\
Excel 2013 - C:\Users\"user name"\AppData\Roaming\Microsoft\Excel\XLStart\
Excel 2010 - C:\Documents and Settings\"user name"\Application Data\Microsoft\Excel\XLStart\
Excel 2007 - C:\Documents and Settings\"user name"\Application Data\Microsoft\Excel\XLStart\
You cannot change any of the above folder locations but it is possible to use another folder as a startup directory.
Alternative Startup Folder
This is a directory that can be used either as an alternative or as an additional automatic startup folder.
If you want to load specific workbooks or add-ins when Excel starts up and do not want to use either of the xlstart directories you can use this folder instead (Excel Options, Advanced)("At start-up, open all files in" folder location on the Options tab.
The alternative startup location is often used to store custom add-ins or a workspace file that you will always want available or open every time you start Excel.
You can also save a workspace file into this folder if you want to open multiple workbooks automatically when Excel starts.