Yes the Companion API was originally designed for the official IOS app but it can also be used by other app. At this time the official IOS app was discontinued but the Companion API stay available for 3rd party tools.
As Gazelle said, you can use EDMC, a tool to send your market data on EDDN a central database for multiple tools:
here some infos about EDDN and connected tools:
- the github wiki
- the thread,
- EDCodex page
As an alternative, since Elite: Dangerous 2.2 you got also the Player log: the game now write dumps with lots of data that can by used by 3rd party app,
(EDMC also use the player journal in addition of the companion API).
You can find lots of app using the Player log.
Take a look to the Codex, there is lot of choice, it will depend of what kind of app you need (trade, exploration...),
you should find an app suiting to your needs.