How to add qml ScatterSeries to existing qml defined ChartView?

should help you out The QML Charts API is written based on the API used by C++ but it is not the same, for example the ChartView is a QQuickItem that does not expose the QChart unlike QChartView is a QGraphicsView(QWidget) that if it exposes the QChart, what Same is with the series. In conclusion you will not be able to use C++(Python) classes to interact with QML.
The example you show at the beginning is not an example for C++ but for QML so it cannot be translated directly into QML. It is also not possible to create a QML Series in C++/Python with QtCharts classes directly, a possible strategy is to use QQmlExpression that can evaluate QML elements and return it to C++/Python. In addition, the createSeries() method not only adds the series but also connects signals.

from enum import Enum, auto
from PySide2 import QtCore, QtGui, QtWidgets, QtQml

# https://code.qt.io/cgit/qt/qtcharts.git/tree/src/chartsqml2/declarativechart_p.h#n105
class SeriesType(Enum):
    SeriesTypeLine = 0
    SeriesTypeArea = auto()
    SeriesTypeBar = auto()
    SeriesTypeStackedBar = auto()
    SeriesTypePercentBar = auto()
    SeriesTypePie = auto()
    SeriesTypeScatter = auto()
    SeriesTypeSpline = auto()
    SeriesTypeHorizontalBar = auto()
    SeriesTypeHorizontalStackedBar = auto()
    SeriesTypeHorizontalPercentBar = auto()
    SeriesTypeBoxPlot = auto()
    SeriesTypeCandlestick = auto()


class DataModel(QtCore.QObject):
    def __init__(self, engine, parent=None):
        super().__init__(parent)
        self.m_engine = engine

    @QtCore.Slot(QtCore.QObject, QtCore.QObject, QtCore.QObject, result=QtCore.QObject)
    def addChartSeries(self, chart_view, chart_axis_x, chart_axis_y):
        context = QtQml.QQmlContext(self.m_engine.rootContext())
        context.setContextProperty("chart_view", chart_view)
        context.setContextProperty("axis_x", chart_axis_x)
        context.setContextProperty("axis_y", chart_axis_y)
        context.setContextProperty("type", SeriesType.SeriesTypeScatter.value)
        script = """chart_view.createSeries(type, "scatter series", axis_x, axis_y);"""
        expression = QtQml.QQmlExpression(context, chart_view, script)
        serie, valueIsUndefined = expression.evaluate()
        if expression.hasError():
            print(expression.error())
            return

        import random

        mx, Mx = chart_axis_x.property("min"), chart_axis_x.property("max")
        my, My = chart_axis_y.property("min"), chart_axis_y.property("max")
        if not valueIsUndefined:
            for _ in range(100):
                x = random.uniform(mx, Mx)
                y = random.uniform(my, My)
                serie.append(x, y)
            # https://doc.qt.io/qt-5/qml-qtcharts-scatterseries.html#borderColor-prop
            serie.setProperty("borderColor", QtGui.QColor("salmon"))
            # https://doc.qt.io/qt-5/qml-qtcharts-scatterseries.html#brush-prop
            serie.setProperty("brush", QtGui.QBrush(QtGui.QColor("green")))
            # https://doc.qt.io/qt-5/qml-qtcharts-scatterseries.html#borderColor-prop
            serie.setProperty("borderWidth", 4.0)
            return serie


if __name__ == "__main__":
    import os
    import sys

    current_dir = os.path.dirname(os.path.realpath(__file__))

    app = QtWidgets.QApplication(sys.argv)
    engine = QtQml.QQmlApplicationEngine()

    dataModel = DataModel(engine)

    engine.rootContext().setContextProperty("dataModel", dataModel)
    file = os.path.join(current_dir, "main.qml")
    engine.load(QtCore.QUrl.fromLocalFile(file))
    if not engine.rootObjects():
        sys.exit(-1)
    sys.exit(app.exec_())

import QtQuick 2.12
import QtQuick.Controls 2.12
import QtCharts 2.2


ApplicationWindow {
    visible: true
    width: 640
    height: 480

    ChartView {
        anchors.fill: parent
        id: bscan0
        ValueAxis {
            id: bscan0_xAxix
            min: 0
            max: 10
        }

        ValueAxis {
            id: bscan0_yAxis
            min: -100
            max: 100
        }
        Component.onCompleted: {
            var serie = dataModel.addChartSeries(bscan0, bscan0_xAxix, bscan0_yAxis)
        }
    }
}

import random
from PySide2 import QtCore, QtGui, QtWidgets, QtQml, QtCharts


class DataModel(QtCore.QObject):
    @QtCore.Slot(QtCharts.QtCharts.QAbstractSeries)
    def fill_serie(self, serie):
        mx, Mx = 0, 10
        my, My = -100, 100
        for _ in range(100):
            x = random.uniform(mx, Mx)
            y = random.uniform(my, My)
            serie.append(x, y)
        # https://doc.qt.io/qt-5/qml-qtcharts-scatterseries.html#borderColor-prop
        serie.setProperty("borderColor", QtGui.QColor("salmon"))
        # https://doc.qt.io/qt-5/qml-qtcharts-scatterseries.html#brush-prop
        serie.setProperty("brush", QtGui.QBrush(QtGui.QColor("green")))
        # https://doc.qt.io/qt-5/qml-qtcharts-scatterseries.html#borderColor-prop
        serie.setProperty("borderWidth", 4.0)


if __name__ == "__main__":
    import os
    import sys

    current_dir = os.path.dirname(os.path.realpath(__file__))

    app = QtWidgets.QApplication(sys.argv)
    engine = QtQml.QQmlApplicationEngine()

    dataModel = DataModel(engine)

    engine.rootContext().setContextProperty("dataModel", dataModel)
    file = os.path.join(current_dir, "main.qml")
    engine.load(QtCore.QUrl.fromLocalFile(file))
    if not engine.rootObjects():
        sys.exit(-1)
    sys.exit(app.exec_())

import QtQuick 2.12
import QtQuick.Controls 2.12
import QtCharts 2.2


ApplicationWindow {
    visible: true
    width: 640
    height: 480

    ChartView {
        anchors.fill: parent
        id: bscan0
        ValueAxis {
            id: bscan0_xAxix
            min: 0
            max: 10
        }

        ValueAxis {
            id: bscan0_yAxis
            min: -100
            max: 100
        }
        Component.onCompleted: {
            var serie = bscan0.createSeries(ChartView.SeriesTypeScatter, "scatter series", bscan0_xAxix, bscan0_yAxis);
            dataModel.fill_serie(serie)
        }
    }
}