Yep. Nobody knows how it works. I've asked.

ToneGym is focused more on ear training rather than music education. If you with to learn the music theory necessary to properly train your ears, please check out the learn section-
https://www.tonegym.co/dashboard/school

@Martha Dahlen : as @Catherine McKay kindly sets clear, MOST people here know how everything works, and MOST would be as nice as Catherine when politely asked the right questions, if you are actually interested...😉

Sometimes Tonegym posts things that help. Awhile ago, they posted some tricks & tips on identifying intervals going up - minor 2nd sounds like Jaws. Major 2nd sounds like Fare A Jacques. Minor 3rd sounds like Greensleeves... ect. I also find it helpful to watch youtube videos explaining things & then I go to the piano and listen.

There are three things you can do outside of the daily workout.

1) Play the same games that are used in the workouts, just on your own time.
2) Do unscored exercises that are similar to those games. Look under "Exercises" on the Gym page.
3) Explore intervals, scales, and the like in a more free-form, self-direct fashion in the tools section. This is non-obvious and easily missed, but it's under the toolbox icon, which is next to "Contests" on the top menu bar.

The site does assume some familiarity with things like interval names (e.g. P5, M3, etc.) and scales. But the site also has the "Learn" tab -- if you are unfamiliar with such things, start with the basic music theory course. It's just a curated list of YouTube videos, but I'd say it is well-curated.

There are also a bunch of other YouTubers that are good teachers. I'm a big fan of David Bennett Piano, for one. He does a good job of connecting music concepts to popular songs, illustrating how something would sound with and without a particular concept, etc. He has a video of how to recognize intervals using popular songs as examples (but there are many other YouTubers with similar videos). Using the opening interval of the Star Wars theme for the perfect 5th and the opening interval of Here Comes the Bride for the perfect 4th will go far in the early levels of interval recognition.

And don't be afraid to "cheat" by having a keyboard alongside to help you pick things out. The games are secondary and a means to an end -- the goal is to help your ear get better. If a keyboard helps you do that, all the better. For the more difficult games like Inversionist and Route VI (recognizing progressions), I still find it essential to use a keyboard.

@Victor Wilburn Very good explanation! Thanks.

@Catherine McKay I really like your summation. very well written.