It's way more comprehensive than just phonics:

- Mandatory screening of students in grades K-3, three times a year, using approved assessment tools, to monitor how early reading skills are developing

- Focused efforts to improve curriculum quality in schools— for phonics and other aspects of literacy

- Large-scale efforts to train teachers

- Retention policies to hold back students who aren’t reading successfully by the end of third grade

Lifted directly from https://www.theargumentmag.com/p/is-mississippi-cooking-the-...

It's also worth saying a lot of the gains evaporate when you factor in learning loss from COVID and Mississippi suddenly screening/helping kids for dyslexia (etc). But, still there's something to learn, just not simply phonics.