Is the System Journal basically the GL Impact that standard transactions create? If yes, then why a search like that doesn't return any results although there are a lot of transactions?

I don't know what a System Journal is. But for sure if you want to get the journal of e.g. Invoice, then you should just choose "Invoice" as the type

Or do it the other way around for testing purposes. Search for a specific

transaction ID

and get its

Type

.