成 grew from a halberd 戊 with a mark of completion: the job is done, the walls are raised, the treaty is sealed. Ancient bronzes use 成 for finished cities and accomplished reigns.
成为 to become, 完成 to complete, 成功 success, 成语 the four-character idioms themselves ('formed phrases'). 成! on its own means 'deal!' in northern China. A child growing up is 长大成人 — growing into personhood.
The halberd rests — work done, walls raised: BECOME, SUCCEED.
成语 idioms are Chinese's compressed hard drive: 万事俱备，只欠东风 'everything ready, only the east wind missing' unpacks an entire Three Kingdoms battle into eight characters.