A Joomla update server supplies information about available extension releases. Its address can be declared in the installed product’s manifest.
What the system reads
Joomla reads release, requirement and download information to offer an appropriate update. The installation archive itself may be hosted at another address. Commercial products can require an active subscription or download key.
When an update is missing
Check the source, enabled state, compatibility, connectivity and developer conditions. No update offer in Joomla does not prove that the latest release is installed. Disabling an update server stops checking that source without uninstalling its extension.
Use trusted Joomla update servers. Do not replace a source address without understanding the origin and purpose of its packages.