You could look up the people who score the highest on binary events, and ask them about their systems. A lot of them are here in the forum (use the search box).
I think most (all?) of the Mongolian competitors use PAO, but that’s just a guess. I think other people there use 9-digits chunks from a 3-digit decimal system (possibly grids), Ben System (10-digit), or binary grids.
From names on that list, Lance told me he uses binary grids. Ola Kåre Risa said he uses the 10-digit Ben System and memorized the images with Anki. Shijir said he uses PAO for cards, but I don’t know about his binary system. I’m not sure if it was mentioned in this video: Why the Shadow System? Shijir Bat-Enkh memorized a deck in 12.74 seconds. He used PAO
Here’s a page about the various techniques: