OwnTracks was MQTTitude. Find the history here on github: https://github.com/binarybucks/mqttitude
OwnTracks uses MQTTClient, a native Objective-C client framework. https://github.com/ckrey/MQTT-Client-Framework
OwnTracks iOS uses CocoaPods (https://cocoapods.org) as a package manager.
All packages are included in the repo, but you'll need to install CocoaPods if you want to modify the pods included (OwnTracks/Podfile).
As with all CocoaPods projects, you need to open OwnTracks/OwnTracks.xcworkspace in XCode, not the .xcodeproj file.