Question: I'm planning to backup a O365 site through PowerShell, but I couldn't find the command for it anywhere on the internet.

Answer: 
There is no powershell command available for backup and restore in SharePoint Online(office 365). 

For backup, you have to rely either to MSFT as they keep the backup for 14 days or you have to use the 3rd party tools to take the backup of your online sites.

<https://threewill.com/the-4-options-you-need-to-know-about-sharepoint-online-backup-and-restore/>

<http://www.metavistech.com/support/how-to/how-backup-and-restore-office-365-and-hosted-sharepoint-environments-using-metavis-ba>
As far as I know—and even after searching for your issue—there isn’t a command for this. There is a script that would help through:
<https://gallery.technet.microsoft.com/scriptcenter/PowerShell-Backup-Script-956f312c>

It is also on GitHub: <https://github.com/Seidlm/PowerShell-Backup-Script>

Still, for enterprises, it is safer to use a third-party solution. There are a bunch of options out there ([Altaro](https://www.altaro.com/), [Nakivo](https://www.nakivo.com/microsoft-office-365-backup/), [Druva](https://www.druva.com/), etc.).