Apple file systems allow a file to have extended attributes or resource forks. Thus a file is not a simple stream of bytes.
When you copy a file to a file system (e.g. FAT) that does not understand these attributes macOS copies those to a ._ (I think if the file system was NTFS then you could probably convert them but I don't think anyone does)
Copying a file out of an Apple environment loses data (OK the data is metadata and usually no one cares)
