以下
いか · ika · JLPT N4 · noun
below
以下 = from (以) + below (下): from this point downward
以 marks a starting point and 下 means down, so 以下 is that point and everything under it. Note it includes the number itself: 10以下 means 10 or less. Its opposite is 以上 (ijō), from this point up.
以 — phonetic 㠯 (plow/tool) + 人 (person)
Originally a pictograph of a hand-held plow or tool (㠯), later combined with 人 (person) to suggest 'using' something. The core meaning 'to use' extended metaphorically to 'by means of', 'with', and 'because of'. In modern Chinese it functions mainly as a grammatical particle.
下 — horizontal line 一 + mark below
下 is an indicative character: a long horizontal line represents a reference level, and a shorter stroke placed beneath it points to the area below. The original meaning 'below, down' comes directly from this spatial diagram, and it later extended to verbs like 'descend' and 'lower'.