There are two types of backups - automatic (backup) and manual (snapshot).
Backups
Project Backups are automatic, periodic captures of the project's geometry. If a project's geometries ever becomes corrupted, you can load a backup to start afresh.
Navigate to main menu (top left) -> Advanced -> Backups
Select the project you want to load from the dropdown.
That project's backups are then listed.
Click Load on the version you'd like to view.
The geometry that was backup up at that save point loads into a new scratchpad.
To save it, create a project boundary, then save.
Only geometry is retained in the project backup. You may import layers and analytics into the new project if you need those as well.
Snapshot
Project Snapshots are packages of everything in the project. You create snapshots manually, unlike backups which are automatic. Use snapshots to save specific versions. Snapshots are saved as Project Attachments.
To create a Snapshot, open Project Settings.
Click "Save Project Snapshot"
The snapshot is saved to the Attachments section.
Click "Open" to view the snapshot in a new tab
Click "Edit" to change the snapshot name and adjust its sharing
Click Load to reload the snapshot in the current window. It loads as an unsaved project (almost the same as Duplicate Project).