擅 almost always appears in compounds: 擅长 (shàncháng, be good at), 擅自 (shànzì, without authorization). On its own it's literary. 擅长 is slightly more formal than 会 or 很会.
mistakes
Don't confuse 擅长 (good at) with 善良 (shànliáng, kind-hearted). Different first character, different meaning entirely.
The hand radical (left-side form of 手) marks 擅 as something done by deliberate human action — a skill exercised, or an authority seized without permission. It sits with 操, 持, 扛 in the broader family of capable, controlling hand-actions.
Lends the sound, drifting from dǎn to shàn. It heads a small phonetic series with 颤 and 檀, all sharing this stem. The classical sincere meaning is unrelated; readers use 亶 here purely as a sound clue for an otherwise opaque character.
