I think this trips up a lot of people, and is also part of a common misconception about Japanese among people who don’t study it. A lot of people have mentioned to me that “their words can sound so many different ways” etc.
It’s important to understand that a word is a word, and that kanji are not words. Kanji are how you write words. A word is simply pronounced how it is pronounced, and written how it is written.
Now, there are guidelines. As had been said, kanji by themselves often use kunyomi readings, jukugo often use onyomi readings, words with okurigana often use kunyomi readings, body parts often use kunyomi readings even in jukugo, etc.
But those are just helpful ways to guess if you don’t know. There are exceptions to everything, and ultimately a word is just a word, and it is read and written as it is. Can you guess how to read the word “weird” in English just by looking at it? Maybe. “Record”? Which syllable has the stress accent? There are some useful guidelines to help you guess. But ultimately, the word is what it is and you just need to know it. You need to think about Japanese the same way.