So I though the way it worked was that a kanji can either exist as separate word like 上 being transcribed as うえ, or as a part of a longer word and then transcribed as じよう, according to the table at wiki:
But every time I see 上 it’s supposed to represent a different set of characters:
In 上る it’s のぼ, and in 上げる and 上がる it’s あ. Is there actually a rule to this and I just stumbled on a bunch of exceptions, or is it the case that it can mean whatever and you need to learn the hiragana for each use of kanji separately?