Spotify’s initial foray into Asia saw it land in small and fairly Western-influenced countries like Hong Kong and Singapore, markets where it was likely to see uptake.
Markets is an appositive, renaming "Hong Kong and Singapore". Where is a subordinate conjunction linking its reference (markets) to the descriptive clause "it was likely to see uptake". The antecedent of it is Spotify.
It means that Spotify started its Asian business in places that where it would probably be successful because those places were commercially Westernized.