The antique prefix arch- comes up in words like arch-nemesis, archenemy, archbishop etc. It means something like “highest” or “supreme”.

✔️ In all these words it’s pronounced with a “ch” sound (like in cheese).

💡 IPA: /ɑɹt͡ʃ/

Audio on Forvo: https://forvo.com/word/arch-nemesis/#en

Unrelated in meaning, but pronounced also with a “ch”, is the word arch and its derivatives like overarching. Forvo

However, more often than not, “ch” in words of Ancient Greek origin is pronounced like “k”: e.g. architecture, archeology, archaic, Plutarch, monarchy.

But not the arch prefix! That one is always pronounced “ch”.

Arch photo by Ulvi Safari on Unsplash