Understanding Application and DB Server Communication Protocols

For example, if the node.js application is running on an EC2 instance and data is exchanged with an RDS instance, what communication protocol would the EC2 instance and the RDS instance be?

It depends on which database engine you choose.For example, if you choose Amazon RDS for PostgreSQL, the PostgreSQL communication protocol is used.

List of database engines that you can select.Frequently Asked Questions - Amazon RDS|AWS

When developing locally, applications and DBs are running on the same host machine, and local differences are also worrisome (such as performance differences).

• Access over the Internet
• Differences in specifications from local host machines
  

I will focus on performance, but RDS is a service from AWS data centers over the Internet, so the infrastructure is completely different and I don't think it's possible to compare it to a local database.

However, there are not many cases where development uses require the same performance as production databases.Even when preparing RDS for development, it is often lower than production specifications for cost reasons.I think it's even better if you build a database on a local host machine.