都 = 者 (a person/thing) + 阝 on the right (the 'city' radical, a shrunken 邑). It first meant a big city, a capital — the place where everyone gathers. From 'all gathered in one place' came its everyday meaning: ALL, every one of them. (As 'capital' it reads dū.)
都 (dōu) is the word for 'all', and like 也 it goes before the verb: 我们都好 = 'We're all fine', 大家都来 = 'Everyone comes'. Read as dū, the same character means 'capital city': 首都 is a nation's capital. One character, two readings, two linked ideas — gathering and the place of gathering.
A great city (阝) where everyone gathers — so 都 means ALL.
都 has two readings: dōu = 'all' (everyday), dū = 'capital city' (首都 = capital). Same character, both rooted in the idea of everyone coming together in one place.