I doubt you can see the availability zone a mysql server is running.
You can't specify the availability zone when creating a read-replica neither via portal nor with PowerShell or Azure CLI. Even if you know in which availability zone the source server is running, it doesn't help.
Sources;
https://learn.microsoft.com/en-us/azure/mysql/howto-read-replicas-portal
https://learn.microsoft.com/en-us/azure/mysql/howto-read-replicas-powershell
https://learn.microsoft.com/en-us/azure/mysql/howto-read-replicas-cli
The purpose of a read-replica is not focussing on high availability. At least that is my understanding.
Source: https://learn.microsoft.com/de-de/azure/mysql/concepts-read-replicas#when-to-use-a-read-replica
Some more information about HA and MySQL are here:
https://learn.microsoft.com/en-us/azure/mysql/concepts-high-availability
Maybe helpful as well;
Flexible Server for Azure Database for MySQL
https://techcommunity.microsoft.com/t5/azure-database-for-mysql/announcing-flexible-server-for-azure-database-for-mysql/ba-p/1686617
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Regards
Andreas Baumgarten