Is this an indirect question or a relative clause?

Question

Jeder Trank gibt euch grundsätzlich 5 Prozent Erfahrungspunktegewinn, was eigentlich schon Grund genug sein sollte, sich regelmäßig einen Schluck zu genehmigen. Um welchen Trank es sich handelt, ist da ganz egal.

So uh, I know welcher can be a relative pronoun and I know you can start a relative clause with any Präposition, so that would check it as being a potential Relativsatz. The problem is that it doesn't feel like it's relative to something to me. There's no information available to me (looked on the internet, or in my books) when an indirect question can start with a preposition, so it's a confusing case to me. I do see it being a Subjektsatz but then the Nebensatz has to be of some type on top of that, right?

Answer 1

It's an indirect question, and it's an subject clause within the main clause.

That um comes from the phrasal verb sich handeln um+‹Akk›. Indirect questions are no different from relative clauses in that regard. They can be lead by a preposition if a phrasal verb requires it.

Sie kennt den Typen nicht, auf welchen sie wartet.

That a phrasal verb in a relative clause.

Sie weiß nicht, auf welchen Typen sie wartet.

This is a phrasal verb in an indirect question.

And it's the same for adverbials:

Sie kennt den Typen nicht, für welchen sie eine Nachricht hat.

Relative clause with an adverbial.

Sie weiß nicht, für welchen Typen sie eine Nachricht hat.

Indirect question with an adverbial.

Answer 2

Yes, an indirect question following common English definition of "indirect question".
But if you want to learn German grammar, you should also know German technical words.

It is important to know that "indirect question" is not the same as "indirekter Fragenebensatz", at least following Duden grammar.

The definition they follow is a Fragenebensatz in Konjunktiv following a verb of questioning, which makes sense in my opinion, because a question is retold by someone else, indirectly.

Going by the same grammar, your examples are Fragenebensätze, but not indirekte Fragenebensätze.
Duden grammar Nr. 1675

I know what you did. and I wonder what you did. are not taken as the same thing in German.

Even just not using Konjunktiv means it is not an indirekter Fragenebensatz anymore:

Er fragte, was er tun solle. (Indirekter Fragenebensatz) Er fragte, was er tun soll. (Kein indirekter Fragenebensatz)