You want to make sure you have the option for preview packages checked. It's located under the Advanced dropdown in the package manager.
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AR Foundation is available as an optional package in the package manager. In the Unity Editor file menu, select Window > Package Manager and select the "Show preview packages" from the Advanced dropdown.
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This shows available packages for import. Select AR Foundation from the list and install it by clicking "Install".
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You should get a similar result.
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