If 17.20 is the active TTU, then we would have:
/Library/Application Support/teradata/client/ODBC/lib -> /Library/Application Support/teradata/client/17.20/lib
/Library/Application Support/teradata/client/ODBC/include -> /Library/Application Support/teradata/client/17.20/include
Inside odbc.ini, all file paths are given in terms of release independent paths which use these symbolic links.
For example, the driver location given in /Library/Application Support/teradata/client/ODBC/odbc.ini is Driver=/Library/Application Support/teradata/client/ODBC/lib/tdataodbc_sb64.dylib which corresponds to /Library/Application Support/teradata/client/17.20/lib/tdataodbc_sb64.dylib if 17.20 is the active TTU.
System DSNs can be found in the odbc.ini files in the default location of /Library/ODBC/.
User DSNs can be found in ~/Library/ODBC/. Alternatively, the ODBCINI environment variable can point to an odbc.ini file present in a non-default location.