Learn Turkish: Bence bu angarya ofisin monotonluğunu artırıyor. - I think this drudgery increases the monotony of the office.

Let's break it down: monoton (monotonous) + -luk (suffix that creates a noun: monotony). Because it belongs to the office (ofisin), it takes the 3rd person possessive -u, which softens the k to ğ (monotonluğu). Finally, we add the buffer -n- and the accusative case -u because this whole phrase is the specific direct object of the verb.

The verb is artırıyor. Does this come from artmak?

Exactly. artmak is an intransitive verb meaning "to increase" (for example, the temperature increases). To say that something raises or causes an increase in something else (transitive), we add the causative suffix -ır to get artırmak.

I often see this verb written as arttırıyor with a double "t". Are both correct?

Officially, no. The standard spelling according to the Turkish Language Association (TDK) is artırmak with a single t. However, you will frequently see native speakers write arttırmak by mistake in informal contexts, so it is very useful to be aware of both.

What exactly does angarya mean compared to just saying iş?