The simple definition of a metro area is an area with at least 100,000 people where at least 50,000 people reside in the core city
However, it's more complicated than that - to find out more go to https://www12.statcan.gc.ca/census-recensement/2021/ref/dict/az/Definition-eng.cfm?ID=geo009
True, although some of the top US cities are deceptive, as they only take in the statistics for the city proper (cities like New Orleans or St. Louis).
Colombia is somewhere quite different.