This single line sets foldmethod to marker. I think this method is better than autocmd method, because filetype checking is not needed, and it only affects 1 file(vimrc), not affecting other vim files.

" vim:foldmethod=marker

Note: this line should be placed within 5 lines of the top or bottom of vimrc file.

Reference:

• dotvim/vimrc at 9804c73c65c04f4550753eb10757f6e01a9940cf · lilydjwg/dotvim
• Modeline magic | Vim Tips Wiki | Fandom