Auntie walked over to the supermarket across the street to buy groceries. Uncle has lived in Taipei the whole time.
界 · Boundary
locative 在 (他在學校)
Pre-verb 在 + place says where the whole event happens (他在家吃飯); post-verb V在 + place says where the verb leaves the subject (他坐在椅子上). Setting of the action vs landing spot of the action.
post-verb 到
到 is motion that arrives at an endpoint (走到門口 — you traveled there); 在 is position settled at a spot (站在門口 — you are standing there). Where you went vs where you are.
阿姨走在超市。 (for reaching it) → 阿姨走到超市。 (motion to a place takes 到, not 在)
叔叔住到臺北。 (for residing) → 叔叔住在臺北。 (a settled position takes 在, not 到)
叔叔在臺北住。 → 叔叔住在臺北。 (the place rides 在 after the verb when 在 names where living lands)
English 'to' and 'in' both come before the place ('walk to the shop', 'live in Taipei'); Chinese hangs both 到 and 在 after the verb, and splits them by reaching vs settling, not by the English preposition.