I want to write a readable Class with my hands (for educational purposes), how to start reading data from a local file?

Question

Node.js

Hello everyone, I'm just learning the node, and it's very difficult to stream...
I want to write something like fs.createReadStream(fileName); Only with your own hands, for a better understanding.
That's actually the question, how to start uploading a file from the local area in chunks? Using the same fs.createReadStream is somehow stupid, since I want to write its likeness, and I want to understand what it uses inside itself, where can I pass the file path so that it starts reading?