How can I add a custom widget when i use a .ui file for my gui?

Question

I'm relatively new to Python. I have a program that does a lot of cool stuff. However, I seem to be restricted with adding in custom widgets because I'm relying on a .ui file to load my entire gui like so:

class Ui(QtWidgets.QMainWindow):
    def __init__(self):
        super(Ui, self).__init__()
        uic.loadUi('images/structure.ui', self)

I want to be able to insert the below entire search widget into a specific part of my GUI.

https://learndataanalysis.org/create-an-instant-search-feature-to-search-data-in-your-table-in-pyqt5/

code:

import sys
from PyQt5.QtWidgets import QApplication, QWidget, QLineEdit, QTableView, QHeaderView, QVBoxLayout
from PyQt5.QtCore import Qt, QSortFilterProxyModel
from PyQt5.QtGui import QStandardItemModel, QStandardItem

class AppDemo(QWidget):
    def __init__(self):
        super().__init__()
        self.resize(1200, 1000)
        mainLayout = QVBoxLayout()

        companies = ('Apple', 'Facebook', 'Google', 'Amazon', 'Walmart', 'Dropbox', 'Starbucks', 'eBay', 'Canon')
        model = QStandardItemModel(len(companies), 1)
        model.setHorizontalHeaderLabels(['Company'])

        for row, company in enumerate(companies):
            item = QStandardItem(company)
            model.setItem(row, 0, item)

        filter_proxy_model = QSortFilterProxyModel()
        filter_proxy_model.setSourceModel(model)
        filter_proxy_model.setFilterCaseSensitivity(Qt.CaseInsensitive)
        filter_proxy_model.setFilterKeyColumn(0)

        search_field = QLineEdit()          
        search_field.setStyleSheet('font-size: 35px; height: 60px;')
        search_field.textChanged.connect(filter_proxy_model.setFilterRegExp)
        mainLayout.addWidget(search_field)

        table = QTableView()
        table.setStyleSheet('font-size: 35px;')
        table.verticalHeader().setSectionResizeMode(QHeaderView.Stretch)
        table.horizontalHeader().setSectionResizeMode(QHeaderView.Stretch)
        table.setModel(filter_proxy_model)
        mainLayout.addWidget(table)

        self.setLayout(mainLayout)

app = QApplication(sys.argv)
demo = AppDemo()
demo.show()
sys.exit(app.exec_())     

Do I have to edit my .ui file to make this happen? I tried creating widgets named search_field and table, and there was no impact. How can I simply create search_field as a qline edit, and table as a TableView or some sort of ListView Widget, and have the above code search properly?

I tried using the above code

edit:

basically, I just want to be able to ctrl + F and search for text in a textBrowser widget.