There are two types of backups available on Platform.sh - manual and automatic.
Automatic backups are taken every day at around 04:00 depending on which timezone a project is based in (the region you selected when creating the project), and roughly every 6 hours after that. Like manual backups, automated backups are stored internally to Platform.sh and can't be downloaded. If you need to be able to download your backup, then exporting your mounts is the best course. Unlike manual backups, automated backups are always live, and don't result in any interruption to your project.
For more information about backups in general, and our backup retention schedule, you can review the following: