I would like to run a periodic (each 4 hour) backup of 'only' the source file of Dynamics Ax 2009, so, the XPO source file.
I'd like to know where they are stored physically.
AX7
All meta data stored as XML and methods in regular files and stored in version control.
See this overview.
AX 2012
The AX source (along with properties and compiled p-code and CIL) is stored in the model store database. In AX 2012 RTM the model store database is the same as the data database (and that is stupid). In AX 2012 R2 the model store database is in a separate database, usually named xxx_Model.
AX 2009 and below
The AX source (along with properties and compiled P-code) is stored in binary files with the AOD extension on the AOS server. There is one file for each layer.
Sorry, source files only do not exist. XPO files is an export/import format only.
The axsys.aod, which is quite large, contains most of the standard code. Corrected elements from service packs are found in the axsyp.aod file.
Your code is most likely stored the axusr.aod or axcus.aod depending on your working layer.
The naming of the files follow a pattern explained here.