How Database Formats Files for Disk

[This SO answer](https://stackoverflow.com/questions/16953638/how-is-mysql-data-stored-on-disk/16953724#16953724) outlines how MySQL organizes its database into files, but doesn't describe the structure of the files themselves. I am wondering, at the physical level, how relational database management systems store their records in files. What the structure of the file is roughly, where the columns/values are (if it's just a CSV), and how it is traversed.