This is part of a series of notes I'm taking as I learn Japanese. While learning you are constantly improving, but when you're experienced you forget what it was like to get there. I'm writing down those thoughts so fluent future me can know the idiot who created him.
I had been studying for 1 year and 5 months when I posted this.
くんよみ is usually used for reading kanji in words with a single kanji as its root. おんよみ readings are usually used for compound words with multiple kanji. I found this completely non-obvious at first. Kanji seem to have several readings with absolutely no rhyme or reason when you start out, but I found myself able to guess how to read some words once I noticed this.
The exceptions seem to work the same way other exceptions in do in 日本語; the obvious reading would conflict with another word, would be be difficult to pronounce, or for no particular reason at all.