Hi Linda,
1.- What do you mean by "type of datasource"?
2.- When you join datasources you choose which is the key between the two datasources. For example, you could use invoice date with approval date to join those two datasources. On the other hand, if you combine datasources the connection between the two datasources is automatic. For example, you combine the datasources account details with account contact data, and when you run this combined datasource it takes automatically the keys between those two datasources.
3.- FINGLAU03 is a standard datasource therefore is not a combinated/join datasource. I recommend you to use "Source Document External Reference" to combine datasources, it going to be so much easier.
Besides, you could check PSM (Public Solution Model) to find technical IDs.
Regards!