C-h v ispell-personal-dictionary shows
nil. Its docstring says:
Documentation: File name of your personal spelling dictionary, or nil. If nil, the default personal dictionary for your spelling checker is used.
My spellchecker is hunpsell, installed in
C:\Hunspell, Windows 10. Emacs finds it and performs spellcheck without issues.
Can't find any personal dic in its folder and subfolders.
Edit: the first comment gives a link, which explains where the default location is.