How do you make save/load system?

Not a game dev yet but it seems like every game has one but the unity does seems to have one built in. So I wanted to understand how it works.

So from the tutorial I saw online they explained the save and load system are basically just programs that either write essential data into a file or read that file and set up the scenes. But I don’t understand how it going to work if you wanted a system with pages of slots available to be save and read.

I only have limited experience with coding so I not quite seeing how it going to work.