Backing up joomla would typically mean:

  1. Copying all files in directory
  2. Copying all relevant databases

However, in our case we are going to be backing up on a Windows, and restoring on an Ubuntu OS. This type of "cross-platform development" presents some unique challenges where differences in operating systems, environments, and configurations come up. For example, trying to restore a backup created by manually copying the files on Windows on Ubuntu could lead to file path errors because the different operating systems don't follow the same filepath conventions and files are installed in different locations. There exist some solutions for cross-platform backup, and we'll try one called Akeeba Backup.

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In your admin panel, navigate to system
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Install > Extensions
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Akeeba backup is very popular, and likely to be the very first option to show up.
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Install
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Navigate to your new extension.
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When run for the first time, a configuration wizard will run automatically.
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The entire process can take a few minutes if you have a slow machine.
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After completion, you can click "Backup Now"
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The backup process can take some time, depending on your customized site and performance of your machine.
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Click on "Manage Backups"
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I suggest that you take this time to download Akeeba Kickstart Core, as it is much more difficult to find later on.
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Place the folder on your Desktop for later use.
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Here is where you would typically be able to download your backups, but in this instance we can see the download button is missing. If you encounter this issue, it is likely a cache or related issue. It can usually be solved by simply logging out and back in. In certain circumstances you may need to clear your browser cache.
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After completing the above, we are now presented with the download button.
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For critical applications, we would want to follow their recomendation. In our case, a regular download is fine.
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Create a new folder named "joomlabackup"
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Place the previously downloaded kickstart.php and your joomlabackup.jpa (renamed) files in this folder and zip it.
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Copy the folder
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Navigate to your www directory
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Paste the folder in your directory
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The backup process is now completed! You can now navigate to this zip folder from Ubuntu and download the backup to restore it on Ubuntu. Note: You will have to know the IP address of your VM to do so. I suggest you run ipconfig in powershell and take note of your IP address.