What are files with the ._ prefix?

Files starting with ._ are created by macOS when copying a file that has extended attributes to a volume that doesn’t support them. This special file is used to save the extended attributes that would otherwise be lost. An example of a volume format that doesn’t support extended attributed is FAT32. These files are hidden when viewing a FAT32-formatted volume in the Finder, but may be shown when you connect it to another device that sees them as separate files.

For more information, see https://apple.stackexchange.com/questions/14980/why-are-dot-underscore-files-created-and-how-can-i-avoid-them