The mixed content fixer changes all references to your images to http into https, thus preventing mixed content issues. It does not change anything in the database, or in your files, it’s a dynamic fix.
The scan in the pro version that you can run before migrating should point out the things the mixed content fixer cannot fix.
Third party embeds will be converted to https, so this will work only if the third party embeds have an SSL certificate on their own domain. It is possible to exclude urls from the mixed content fixer, but then you will have mixed content of course.