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Your module URL will load based on the "BaseUrl/frontName/controllerName/actionName"
So as per your module code,
frontName = unit3helloworldblock
controllerName = Block
actionName = Index
So the URL of your module is
https://magento2.mgt/unit3helloworldblock/block/index
You can also check on this Magento DevDocs: https://devdocs.magento.com/videos/...
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