Nodejs STDIO duplexes gibberish

Question

Quite confused discovered (on nodejs example, not sure about other langs yet) lately that all three stdio streams are a full duplexes (i.e. both readable and writable).

I'm not sure whether the first my question should be the why. Yet it might be helpful to solve the puzzle as well.

From the other side what really stands behind all the other directions. E.g. what supposed to mean writing into the stdin itself.

But the main question is on top of all these is about the following consequences.

I have this as a basis:

const {Transform} = require('stream')

// just an util to visualize the source
const streamLogger = (name) => new Transform({
    transform(chunk, encoding, callback) {
        callback(null, `--------${name} ${chunk}`);
    },
})

Starting with a simple

process.stdin.pipe(streamLogger('stdin')).pipe(process.stderr)

node index.js
>>>abc
<<<--------stdin abc
>>>def
<<<--------stdin def

Quite as expected, so far

process.stdin.pipe(streamLogger('stdin')).pipe(process.stderr)
process.stdout.pipe(streamLogger('stdout')).pipe(process.stderr)

node index.js
>>>abc
<<<--------stdout abc
>>>def
<<<--------stdout def

So why the stdin is just switched to stdout regardless stdin was even piped (subscribed) in the first place?

And even more interesting

process.stdin.pipe(streamLogger('stdin')).pipe(process.stderr)
process.stdout.pipe(streamLogger('stdout')).pipe(process.stderr)
process.stdin.write('hello\n')

node index.js
<<<hello
>>>abc
<<<--------stdout abc
>>>def
<<<--------stdout def

Why is hello even printed bypassing the logger at all?

So what's really going on over here?

PS: oh - just discovered even more strange.

Once changed the pipes order (stdout goes first)

process.stdout.pipe(streamLogger('stdout')).pipe(process.stderr)
process.stdin.pipe(streamLogger('stdin')).pipe(process.stderr)

It just competes for the data like

node index.js
>>>abc
<<<--------stdin abc
>>>def
<<<--------stdout def
>>>ghi
<<<--------stdin ghi
>>>jkl
<<<--------stdout jkl

Please help to make sense of this.

Thanks