interface LayoutModifier : Modifier.Element {
    fun MeasureScope.measure(
        measurable: Measurable,
        constraints: Constraints,
        layoutDirection: LayoutDirection
    ): MeasureScope.MeasureResult

How exactly LayoutModifier works? instead of list of measurables for each child, there is only one measurable. and I've playing with it, when I tried

placeable.place(50.ipx, 50.ipx)

it applied for all children. I think there is some thing I misunderstood about it.

There is only one child of a modifier: the element it is applied to

Thanks @Adam Powell but sorry I can't understand your explanation Let me explain what I'm struggling with:

@Composable
fun SimpleBox(modifier: Modifier = Modifier, children: @Composable() () -> Unit) {
    Layout(children = children, modifier = modifier) { measurables, constraints, layoutDirection ->

        val placeable = measurables[0].measure(constraints)
        val placeable2 = measurables[1].measure(constraints)
        layout(placeable.width, placeable.height) {
            placeable.place(0.ipx, 0.ipx)
            placeable2.place(0.ipx, 100.ipx)
        }
    }
}

private class SimpleLayoutModifier(): LayoutModifier {
    override fun MeasureScope.measure(
        measurable: Measurable,
        constraints: Constraints,
        layoutDirection: LayoutDirection
    ): MeasureScope.MeasureResult {
        val placeable = measurable.measure(constraints)

        return layout(placeable.width, placeable.height) {
            placeable.place(50.ipx, 50.ipx)
        }
    }
}

Only the SimpleBox is affected by it. The modifier acts on the SimpleBox's layout node and is not aware of what the SimpleBox contains

AH, that was my first guess but when I saw the blue color not moves and it just stays there, I thought it's wrong. Why the blue Box not moves?

Yes, each modifier is applied in order. A modifier applies to everything that comes after it in sequence