myTracks supports iCloud synchronization. That means, it synchonizes the tracks between all iOS and macOS devices. However, the files representing the tracks are stored in a hidden part of iCloud Drive. Therefore these files are neither visible in the iOS Files app nor in Finder on macOS. In this regard myTracks behaves similar to Apple's Photos app. The photo files of Photos are also not directly visible.
From a technical point of view on macOS the files are located at the following location within your home folder:
Library/Mobile Documents/4TNB35NY7G~info~stichling~myTracks. Nevertheless, please do not modify these files manually because this can result in unpüredicted behavior.