艸 is the pictograpgy (象形文字) of grass when the word was created back in ancient time. 草 is the character of "grass" in Chinese The third one is radical (部首) for characters relating to plants in both traditional and simplified Chinese. The last one is the same radical only used in simplified Chinese For the third and fourth ones, they are parts of characters. You cannot use them independently.

The 艹 on top of 釆 (to pluck) is the grass radical. It is one of those compound words that uses 艹 as the semantic component while 釆 forms the phonetic component. It is NOT a character that is composed by inference or logic as in 休 (man under the tree to mean "rest"). The 䒑 at the bottom of 豆 is not the grass radical. The 艹 radical always appear on the top but never at the bottom of a radical (at least I can't think of any at the present moment). Due to abstraction and simplication, it just looks like it. If you really want to know what it represents, you have to look at the oracle bone script. For example, 立 (to stand) the bottom part is made up to human legs standing on the ground represented by the horizontal stroke. The top part, by the way is the head of a man with his arms outstretched.

Except for 草, all are variants of the same radical 艸 for plants other than trees. These variants are used in Chinese calligraphy depending on the script used. 艸 is used for seal scripts, 艹 is the Sung Dynasty standard print font for clerical script (Kaishu or the standard script) and the last one is used in grass hand script. As an example, see 0:52 of https://www.youtube.com/watch?v=e3lwCLvaHmg for writing 花 in the grass hand.

actually，we only use the second charactar 草，and it means grass，but the left three words，tbh，ive never seem them before and it seems that no one use them in writting and speaking.

我也不知道第一个汉字是什么[emoji]