Several parts combine into one character.

因 is a compound ideograph: □ (an enclosure / mat outline) + 大 (a large figure of a person, arms and legs spread). The composite picture is concretely physical: a person reclining or resting on a mat — relying on it, depending on it. The original meaning was actually "mat / sleeping pad," but the idea of "what one rests on / depends on" abstracted into "cause / reason / what something depends on." The etymological insight is profound: a cause is what an effect rests on, what it depends on for its existence. Buddhist philosophy adopted 因 for this exact metaphysical sense.

Korean reading "in." 原因 (wonin, cause — top-frequency Korean noun in formal writing), 因果 (ingwa, cause and effect — used in Buddhist contexts and in scientific reasoning), 因緣 (inyeon, karmic connection / fated relationship — central Korean spiritual concept), 要因 (yoin, contributing factor / chief cause), 因子 (inja, factor — used in mathematics and biology).

Mandarin yīn, 1st tone. 因 (yīn), 因为 (yīnwèi, "because" — one of the highest-frequency conjunctions in spoken Mandarin; introduces causal clauses), 原因 (yuányīn, cause), 因此 (yīncǐ, "for this reason" / "therefore"), 因素 (yīnsù, factor / element). 因为 is essential vocabulary from week one of Mandarin learning; almost every causal explanation in Chinese opens with 因为...所以... ("because... therefore...").

Japanese on-reading イン (in) — 原因 (gen'in, cause), 因果 (inga, cause and effect — including the Buddhist sense of karmic causation), 要因 (yōin, main factor), 因縁 (innen, karmic / fated connection — used in dramatic Japanese contexts: "this rivalry was destined" / "we have an old score to settle"). Kun-reading よる (yoru, "to be due to / be caused by") — appears in formal compounds: ~に因り (~ ni yori, "due to ~") in legal and formal documents. The pair 因 / 縁 (in / en) frequently appear together — 因縁 means specifically the karmic causal links that bind people across lifetimes in Buddhist cosmology.

Memory aid: a person (大) lying on a mat (□) — what you rest on / depend on = cause.