How to update the databse replicas?

Does the updates happen automatically between the replicas? I mean, the databases like mysql/mongo/cassandra are handle that? or the programmer needs to?

Yes nearly all databases support replication, it happens automatically but you need to make certain decisions how the replication should happen. A very good read to explain a lot about replication:

Here is some relavent except from this link:

When we talk about replication, we are basically saying that when I write some data in a given node A , this same data also needs to be written in node B (and maybe C and D and E and…), but we need to decide how this replication will happen, and what are the guarantees that we need. As always, it’s all about trade-offs.

Poor article on data replication by grokking. Atleast have basic definition and types of replication defined in your article. What’s the point if someone has to ask this basic question and go out and read a 3rd party article