Ten digits, ten sounds, learned once.
Each digit keeps one family of consonant sounds, and vowels are never assigned, so you drop them in wherever they make a real word. Most of the pairings have a reason baked in (the hook under each card), so it isn't ten arbitrary facts.
Three quick checks.
Which sound does 3 keep?
Two sounds become one word.
A lone "k" isn't a picture. Pair two digits and their two sounds make a consonant skeleton a real word can hang on, and that word is the picture. The story's pairs work the same way: LION is L·N, BEACH is B·CH.
Two digits walk on stage.
The code doesn't care whether you've met the number before: 34 is M and R, and MOWER is sitting right there. Any pair works this way, so 52 could just as easily be LINE instead of LION and 58 LOAF instead of LAVA - use whichever real word actually sticks.
Type any two digits.
You get the two sounds plus the starter word the Memory Mill app uses for that pair. Don't love it? Any real word with those two sounds works. Want all hundred at once? The full 00 to 99 chart is one page.