If you have a farm backup. How can you restore just one site or site collection using powershell / central administration please?
SharePoint farm backups allow you to target individual items within the backup for a restore, but you can't get any more granular within the backup than a content database.
When you restore that backup with Central Administration, you should see checkboxes next to the individual items that you can restore to your environment. With PowerShell, you first need to run Restore-SPFarm with the -ShowTree parameter, which will tell you what components are included within the backup you're trying to restore as well as the unique identifiers for each of those components that you'll need to pass through with the -Item parameter to restore the indvidual component.
The only ways you can target an individual site collection within a farm backup is if you either have a single site collection inside a content database or if you first restore to a recovery farm and use SharePoint's site collection backup to target the site collection you want.