Das Onlinelexikon erklärt auch, in welchem Kontext eine Redewendung benutzt wird.

The preposition in can take either dative or accusative:

• Dative = location / state (where something is)
• Accusative = direction / movement (where something is going)

Here, Kontext is not a direction but a situation or background in which something happens, so it’s a static location in an abstract sense: in which context → dative.

• Kontext is masculine.
• Masculine dative singular of welcher = welchem.
• Masculine dative singular of Kontext stays Kontext (no extra ending).

So:

• in welchem Kontext = in which context (dative → location)
• in welchen Kontext = into which context (accusative → movement, would sound wrong here)

auch means also / too / as well.
Here, it shows that explaining the context is an additional function of the online lexicon, besides other things it already does.

• Das Onlinelexikon erklärt auch, ...
  = The online dictionary also explains ...

Position: auch is placed in the “middle field” of the main clause, after the verb erklärt would be unusual, so it goes after the subject and before the verb:

• Subject: Das Onlinelexikon
• Verb (2nd position): erklärt
• Middle field: auch
• Subordinate clause: in welchem Kontext ...

You could move auch a bit, but then you would change the emphasis:

• Auch das Onlinelexikon erklärt, ...
  Emphasis on the fact that the online lexicon, too (in addition to someone/something else), explains this.

The sentence has:

• a main clause: Das Onlinelexikon erklärt auch
• followed by a subordinate clause: in welchem Kontext eine Redewendung benutzt wird

In German, subordinate clauses are always separated from the main clause by a comma.
The subordinate clause is introduced by a question word (welchem as part of in welchem Kontext) and functions as the object of erklärt (it tells us what is being explained).

So the comma marks the boundary:

• Das Onlinelexikon erklärt auch, [in welchem Kontext eine Redewendung benutzt wird].

It is an indirect question clause (in German: indirekter Fragesatz), also called an embedded interrogative clause.

• It is introduced by a question phrase: in welchem Kontext.
• It has the typical subordinate-clause word order (finite verb at the end): ... benutzt wird.
• It acts as the object of erklärt:
  What does the online dictionary explain? → in welchem Kontext eine Redewendung benutzt wird.

So the structure is:

• Main clause: Das Onlinelexikon erklärt auch
• Object clause (indirect question): in welchem Kontext eine Redewendung benutzt wird

In main clauses, German has verb-second word order:

• Main clause: Eine Redewendung wird benutzt.
  (finite verb wird in second position)

In subordinate clauses, the finite verb goes to the end of the clause:

• Subordinate clause: ... dass eine Redewendung benutzt wird.

In the sentence you gave, the part eine Redewendung benutzt wird is a subordinate clause, so:

• Subject: eine Redewendung
• Past participle: benutzt
• Finite verb (auxiliary) at the end: wird

Hence: eine Redewendung benutzt wird is the normal subordinate-clause order.

benutzt wird is:

• Present tense (Präsens)
• Passive voice (Vorgangspassiv)

Literal structure:

• auxiliary verb werden (here: wird) + past participle benutzt
  = is used

The active version would be something like:

• jemand benutzt eine Redewendung
  (someone uses an idiom)

In the given sentence, passive is chosen because we don’t care who uses the idiom; the focus is on the idiom itself and the context in which it is used.

Yes, you can say:

• in welchem Kontext eine Redewendung verwendet wird

benutzen and verwenden both often mean to use and are largely interchangeable here.

Nuance:

• verwenden can sound a bit more formal or technical.
• benutzen is very common and neutral, sometimes slightly more everyday.

In this context (describing what an online lexicon explains), both are perfectly fine and natural.

eine Redewendung is indefinite singular and is used here in a generic sense: it doesn’t mean one specific idiom, but idioms in general.

• eine Redewendung
  = an idiom / an expression (any idiom, generically)

Alternatives and their nuance:

• die Redewendung
  Would refer to a specific, known idiom (for example, one that was just mentioned earlier).

• Redewendungen (plural, no article)
  Would stress the plurality: idioms in general, perhaps more strongly highlighting that there are many.

So eine Redewendung is a very natural way to talk about the concept of an idiom in general.

German loves compound nouns. Onlinelexikon is a compound of:

• Online (describing the medium)
• Lexikon (encyclopedia / lexicon / reference work)

Rules:

1. Compounds are usually written as one word and capitalized: Onlinelexikon.
2. The last element determines the gender of the whole compound.
   
   • Lexikon is neuter → das Lexikon
   • Therefore: das Onlinelexikon

You might also see Online-Lexikon with a hyphen; that’s also acceptable. But as a solid compound, Onlinelexikon with das is standard and correct.

All are reference tools, but with slightly different focuses:

• Wörterbuch
  = dictionary (focus on words, spelling, meanings, translations, etc.)

• Onlinewörterbuch
  = online dictionary (same as above, just on the internet)

• Onlinelexikon
  = more like online encyclopedia / online reference work
  It suggests broader explanations: not only what a word means, but also background information, usage notes, examples, context, etc.

For idioms and explanations of context, Onlinelexikon sounds very natural, because an encyclopedia-style resource typically explains usage in more depth than a simple word list.

You could form related sentences, but the meaning changes:

• wo = where (place)

  • Das Onlinelexikon erklärt auch, wo eine Redewendung benutzt wird.
    → The online dictionary also explains where an idiom is used.
    Focus on physical or situational places (e.g. in which region / in which medium).

• wann = when (time)

  • Das Onlinelexikon erklärt auch, wann eine Redewendung benutzt wird.
    → ... explains when an idiom is used.
    Focus on time-related situations (e.g. in the past, nowadays, in formal situations, etc.).

• in welchem Kontext = in which context
  This is broader and more abstract: social, stylistic, situational context (formal vs informal, written vs spoken, joking vs serious, etc.).

So in welchem Kontext is more about situational/functional usage than about concrete time or place.

Yes, but it changes the meaning:

• Original:
  Das Onlinelexikon erklärt auch, in welchem Kontext eine Redewendung benutzt wird.
  → The explaining is what’s “also” being done.

• With auch in the subordinate clause:
  Das Onlinelexikon erklärt, in welchem Kontext auch eine Redewendung benutzt wird.
  Now auch attaches to eine Redewendung:

  • It suggests that in that context, an idiom is used as well, in addition to something else (maybe other types of expressions, other ways of speaking).

So:

• auch in the main clause → also explains
• auch before eine Redewendung → an idiom is also used (among other things)

The original sentence wants to say that explaining the context is an additional function of the online lexicon, so auch correctly stays in the main clause.