I don't quite understand what you mean about your kana description, but it's important to know that while it's good to know the individual meaning of kanji, it's far more important to know how they're used, and what common compound words they show up in.
了 is very common in Chinese, because it's used to mark certain grammar structures, like past tense and imminent actions. In Japanese, I don't think you'd ever see this character used by itself, and it doesn't even have kunyomi, so it's only used as part of a kanji compound. A common one I like to use (kinda jokingly) is the semi-military term 了解, "understood," which is used to affirm orders.
終 does have kunyomi and can be used on its own: 終わり. This is a common word for "ending," and many times you'll see 終 by itself at the end of an old movie or something, and it would be read the same: owari. It's also used in a lot of other compounds and is way more common (jisho.org has 133 hits for this character compared to 22 for 了).
One of those compounds is actually 終了 which has more or less the same meaning as 終わり, but again, it's more important how it's used. The more you read, watch, and consume things in Japanese, the better a grip you'll have on these characters and how they're used.