What DSN file is MS Access using for ODBC Linked Tables?
IIRC, Access uses either a machine DSN or a file DSN. I only have Access 2016 installed but I don't think this has changed much. There should be an External Data tab in the ribbon, click on ODBC Database. This shows you two tabs: File Data Source and Machine Data Source. 


This image shows the MS Access Database source but I generally see folks create one specific to the Access database they created.

You can also views this by opened the ODBC Data Sources from Administrator tools. There is a 32 and a 64 bit version of this and you may need to check both.

Also, with regards to Access storing the connection itself. It is possible to setup a DSN-less connection using macros. You can see this KB on how that is done to know where to check in your Access database.