Coldcard is using called Boot ROM, according to them this is piece of code stored in ROM, but they are hiding the fact that this code is totally closed source, you can't even view it.
Interesting find. There isn't much information about it when searching online. They have a section on their docs page that explains what Boot ROM is, but not much information there. Perhaps it's possible somehow to view the Boot ROM code by downloading the firmware files from
here. The file descriptions mention that Boot ROM is part of the file. The size of the files containing Boot ROM code is larger than the files without it. Maybe someone knows how to analyze it if it's viewable at all...