Allow me to be as strigthforward with my answers as you are in your questions.
How would a microservice architecture work in a Node.js CRUD server,
does a simple CRUD server even benefit from microservices?
This is a wrong question. The Microservices architecture is not a technical answer to a technical question. It's rather a technical strategy for organizational needs.
Don't ask what Microservices can do for your actual application. Do ask what can they do for your company. For your business.
They provide the company with the capacity to deliver new services to the customers quickly and directly. To adapt your business to the changes of the market. In a constantly changing world, that could be an unvaluable capacity.
However, it's not for free. It might interest you to take a look to the trade-offs.
What are the kinds of things that commonly get delegated to a
Business capabilities. Often referred as bounded contexts. This's a very broad and complex subject. For further references, seek out by: Microservices decomposition strategies.
Is a microservice the same as a module in a program or does it have a
completely separate process?
They are completely separated processes. Microservices are independent in almost all the senses.
How do microservices communicate with the main server, is it something
like UNIX sockets?
There's no centric components (server) in the Microservices architecture. That goes totally against its nature. As @scriptin commented, Microservices are stand alone applications. Small applications working together. A Microservice is both client and server at the same time.
Allow me to do a naive comparision. The Microservices philosophy is cooperativism. They work like a soccer team. Microservices (players) cooperate with each other for a greater good.