Read replicas
Scale the read workload of your database. Read replicas are additional database instances that replicate data from the primary RDS instances in real time. Your application can read data from the replicas also, to distribute and offload read traffic from the primary instance, improve overall database performance.
When writing data, the data in only written to the primary database instance.
Multi-AZ
Multi-AZ deployments are designed to enhance high-availability by automatically replicate your database to a standby instance in a different Availability Zone. It provides the ability to failover in case of AZ outage.
Data is only read and written to the primary database instance, the failover database is not accessible unless the failover is triggered. You can only have one other AZ as failover AZ.
Multi-Region (read replicas)
Multi-Region deployments allow you to replicate a source RDS instance to a target region. If you have applications in other regions, Multi-Region deployments provides data locality for improved read speed, any time the application needs to access data it will access the local database. When it comes to writing data, the data are written to the primary database.
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Reference* - Udemy Ultimate AWS Certified Cloud Practitioner CLF-C02