Learn Hardcore German: Die Behörde verlangt, dass wir unsere Dokumente irgendwo online einreichen, wofür wir eine sichere Datenbank benutzen. - The authority requires that we submit our documents somewhere online, for which we use a secure database.

verlangen is one of those German verbs that require the subordinating conjunction dass to introduce a clause. You cannot say verlangt unsere Dokumente einzureichen; instead you need verlangt, dass wir … einreichen. In English you can say “requires us to submit,” but in German you must use dass plus a finite verb at the end.

Why is the verb einreichen placed at the end of its clause?

In German subordinate clauses (like the one introduced by dass) follow the verb-final word order. The finite verb (verlangt) stays in second position in the main clause, but in the dass-clause the main verb (einreichen) moves to the very end.

What does wofür mean here and why is it used?

wofür is a relative adverb combining wo + für, meaning “for which” or “for that purpose.” It refers back to submitting documents online and introduces a subordinate clause that explains the purpose: “for which we use a secure database.”

Could you replace wofür with dafür or für die?

Not directly as a relative connector.

dafür means “for that,” but it doesn’t introduce a clause. You could split it into two sentences:
“Die Behörde verlangt, dass wir … einreichen. Dafür benutzen wir eine sichere Datenbank.”
für die would need a noun: für die Einreichung (“for the submission”) and change the structure:
“…, wo wir eine sichere Datenbank für die Einreichung benutzen.”

What case is implied by wofür?

The preposition für always takes the accusative case. Although wofür is an adverbial connector, it corresponds to für + accusative underneath, so think of it as “for which (accusative).”

Why is unsere Dokumente in the accusative plural form?

Dokumente is the direct object of einreichen, so it must be in the accusative case. unsere is the possessive determiner for “our” and agrees with Dokumente in number (plural) and case (accusative), giving unsere Dokumente.

What’s the nuance between irgendwo online and irgendwo im Internet?

Both mean “somewhere online,” but

online is an adverb borrowed from English, more colloquial.
im Internet is a native German prepositional phrase, slightly more formal. You can use either without changing the meaning.

Can you use hochladen instead of einreichen?

You could if you only want to say “upload.”

hochladen = “to upload” (technical).
einreichen = “to submit” (official).
Since an authority verlangt an official submission, einreichen is the more appropriate verb here.