Here's how it's implemented:

class Block(QFrame):
    def __init__(self, parent):
        super().__init__(parent=parent)
        self.setObjectName("Block")
        self.setStyleSheet("QFrame#Block {background-color: rgba(0, 0, 0, 0.1);} "
                           "QWidget#InnerWidget {background-color: rgba(0, 0, 0, 0.1);}")
        self.main_layout = QVBoxLayout(self)

    def add_button(self, name):
        w = QWidget(self, objectName="InnerWidget")
        l = QHBoxLayout(w)

        l.addWidget(QLabel(name, parent=w))
        l.addWidget(QPushButton(name, parent=w))
        self.main_layout.addWidget(w)

    def add_text_area(self, name):
        w = QWidget(self, objectName="InnerWidget")
        l = QVBoxLayout(w)
        title_widget = QWidget(w)
        title_layout = QHBoxLayout(title_widget)
        title_layout.addWidget(QLabel(name, parent=w))
        title_layout.addWidget(QLabel(name, parent=w))
        l.addWidget(title_widget)

        text_edit = QPlainTextEdit(parent=w)
        l.addWidget(text_edit)
        self.main_layout.addWidget(w)


class Column(QWidget):
    def __init__(self, parent):
        super().__init__(parent=parent)
        self.setObjectName("Column")
        layout = QVBoxLayout(self)
        layout.setSpacing(20)
        self.setStyleSheet("background: transparent;")

    def add_block(self, block):
        self.layout().addWidget(block)


class RootWidget(QFrame):
    def __init__(self):
        super().__init__()
        self.setObjectName("RootWidget")
        self.setStyleSheet("background-color: rgb(80, 80, 80); color: white;")
        self.setAutoFillBackground(True)

        layout = QHBoxLayout(self)
        self.main_widget = QScrollArea(parent=self)
        self.columns = QWidget(parent=self.main_widget)
        self.columns_layout = QHBoxLayout(self.columns)

        self.column_1 = Column(self.columns)
        self.column_2 = Column(self.columns)
        self.column_3 = Column(self.columns)

        for column in [self.column_1, self.column_2, self.column_3]:
            for i in range(3):
                block = Block(column)
                for j in range(10):
                    block.add_button(str(j))
                    if i == 1 and j == 9:
                        block.add_text_area("TEST")
                column.add_block(block)
            column.layout().addStretch()
            self.columns_layout.addWidget(column)

        self.main_widget.setWidget(self.columns)
        layout.addWidget(self.main_widget)

This is what I end up with:

Problem: I can't resize QPlainTextEdit in any way. How can this be done?

I have tried resize(), setGeometry(), however nothing has worked.

def add_text_area(self, name):
    w = QWidget(self, objectName="InnerWidget")
    l = QVBoxLayout(w)
    title_widget = QWidget(w)
    title_layout = QHBoxLayout(title_widget)
    title_layout.addWidget(QLabel(name, parent=w))
    title_layout.addWidget(QLabel(name, parent=w))
    l.addWidget(title_widget)

    text_edit = QPlainTextEdit(parent=w)
    text_edit.resize(100, 1000)
    # or text_edit.setGeometry(text_edit.geometry().x(), text_edit.geometry.y(), 100, 1000)
    l.addWidget(text_edit)
    self.main_layout.addWidget(w)