What in particular? The gameplay or the games linking in together? I think it was originally using the ini object and reading values, but as the new save files can't be opened in notepad they may have moved to the array object or something else. Its fairly simple to do. At the start of the game it looks for the mr tomato and ms lemon save files, read the values from them and then displays the frames accordingly.
You haven't even looked into what I was doing - Pikuniku 2 was being made with permission. And you said to me "Are you familiar with copyright infringement? At best you can develop a fan game, but judging by your profile you don't have anywhere near the skills to do that either." Never judge a book by its cover, and I naturally assume that you haven't even downloaded a single one of my games.