Recently, I have been studying kanji seriously. I just learned the compositions 成り立ち of kanji. There is a 成り立ち that confuses me a lot. My text book says 池 is a kanji categorized as 形声文字 (kanji composed with 音記号（音符） and 意符. 音記号 of 池 is 也, but 也 itself is read ヤ not チ（onyomi of 池）or いけ. How come this kanji 池 is read チ（音）. Is 音読み and reading of 音記号 are completely different things? However, is there a possibility that my textbook (my professor's hand-made book) is wrong.
The answer is clearer if we look at how they may have been pronounced in Old Chinese.
- (Baxter–Sagart): /*Cə.lraj/
- (Zhengzhang): /*l'al/
- (Baxter–Sagart): /*lAjʔ/
- (Zhengzhang): /*laːlʔ/
(Baxter-Sagart refers to one set of reconstructions, specifically the one in this book.)
Now, reconstructions are necessarily based on incomplete information, and so you can see that the two reconstructions don't agree on what the initial or final of 池 and 也 actually are. However, both of them agree that the two characters were certainly pronounced more similar to each other than in modern Chinese varieties.
You can see that 池 ended up diverging from 也 in pronunciation starting from Middle Chinese because of a difference in articulation in the initial (either a sesquisyllable in Baxter-Sagart, or a pharyngealised initial in Zhengzhang's reconstruction).
In summary, you can see that while 也 is not a meaningful phonetic component from the point of view of modern Chinese dialects or Sino-Japanese readings, they were much closer to each other in Old Chinese.
(PS: Your question also applies to 地, which is certainly more similar to 池, but also raises the similar question of why it has a phonetic 也. The answer is the same as above.)
While the history of Japanese characters being imported from Chinese is relevant, it is probably more useful to focus on how Japanese is classified now.
Kanji characters are usually categorized into 4 types, as follows:
- Pictograms (象形文字) like 門
- Ideograms (指示文字) like 二 or 三
- Compound ideograms (会意文字) like 明
- Phonetic-ideographic characters (形成文字) like 池
(There are 2 other categories but they are somewhat rare. )
The phonetic-ideographic characters contain a component that sometimes gives a clue to the pronunciation of the character. For example, you will find that the component 昜 sometimes appears on the right side of a kanji. As you can see here, it usually carries the pronunciation of ヨウ. But not always. Therefore, as a learner you can't rely on specific components always being pronounced a certain way. Kanji evolved in a very unorthodox way, meaning that learners are better served by developing an awareness of the components, but not relying on them to provide solid rules about meaning or pronunciation.
If you want to check the category of any joyo kanji, try this site: https://www.kanjidatabase.com/kanji_search.php and select 'Look Up Kanji' and then tick 'Kanji Classification'. Don't let this derail your learning process - it is more relevant in academic discussions, although it can have implications for learners.