Windows 10 stores its user-defined dictionaries in %AppData%\Microsoft\Spelling. Inside this directory, there are language-specific subdirectories (such as en-US or en-UK). Within each of those, there are text files; default.dic is the custom dictionary file for the spellchecker and default.acl is the AutoCorrect file.
The custom dictionary file contains a simple list of words, one per line. For example:
The AutoCorrect file is slightly more complicated and contains pipe-delimited lines:
adn | and teh | the
Each line consists of spelling to be automatically corrected, the pipe character, and the correct spelling.