Single-character words in Chinese are like single-word English words. They have to be learned and memorized individually, and these words can join together and form a compound word that also needed to be memorized individually
Words like "chainsaw" are compound words that have to be memoized individually
If you have learned the words "chain" and "saw", it is easy to form a mental image of the two concepts together and remember the word "chainsaw"
Words like 長頸鹿 are also compound words that have to be memorized individually
When you know the characters 長, 頸, and 鹿, it is easy to form a mental image of a long-necked deer that resumable a giraffe
There's no shortcut.
To remember the word shortcut easily, you must know the words "short" and "cut"
It is the same for 捷徑. knowing the characters 捷(quick) and 徑(path) make it easy to remember the word 捷徑 (quick path) which translated to "shortcut"
同 = same
意 = idea
同意 (same idea) is officially translated as "agree" and you have to remember it
合 short for 聯合 = join
同 short for 同意 = agree
書 = book/ document
合同書 means "contract" and it is shortened to "合同" in Chinese.
Now you've learned a new word "合同"(contract), thanks to your prior knowledge of 聯合 and 同意
How do you know when you are using compound words vs. a sequence of simple words in Chinese?
You know you are not using compound words when these words do not form a compound word that you can find in the dictionary
For example, 直接 is a compound word for "direct" but 直說 is short for 直白地說 (talk straight and honestly --> not talk in a circle), and they are two words (adv + v)