If I'm reading this right
By 'site', you mean site collection, correct? (Just making sure)
https://learn.microsoft.com/en-us/sharepoint/administration/back-up-site-collections
https://learn.microsoft.com/en-us/powershell/module/sharepoint-server/backup-spsite?view=sharepoint-ps
If it's an spweb:
https://learn.microsoft.com/en-us/sharepoint/administration/export-a-site-list-or-document-library
Sounds like you'd be using something like
Backup-SPSite https://intranet.contoso.local/sites/site_name -Path C:\Backup\site_name.bak
I noticed it's an unattached db. Is the site collection listed when you run
get-spsite
?