Each folder in `$MAINIMG` is a Magisk module, except the folder `.core` which stores files that are unrelated to any modules.
```
# The directory storing all non-module files
COREDIR=$MOUNTPATH/.core
# MagiskHide hidelist
HIDELIST=$COREDIR/hidelist
# Folder storing scripts that should be executed in post-fs-data mode
POSTFSDATA_DIR=$COREDIR/post-fs-data.d
# Folder storing scripts that should be executed in service mode
SERVICE_DIR=$COREDIR/service.d
```
Each folder in `$MAINIMG` is a Magisk module. There used to be a folder `.core`, which is no longer used since v18.0.
### Final Words
The file structure of Magisk is designed in a weird and overly complicated way. But all of these quirks are done to properly support hiding modifications from detection. These design choices are mostly what makes Magisk difficult to implement properly and maintain.