$ cd /home/ubuntu/
$ mkdir -p hoge/test # hogeアプリのディレクトリ、各種コードはhoge/testに格納する
$ cd hoge
$ python3 -m venv venv
$ source venv/bin/activate
$ pip install flask mod_wsgi
$ vim flaskapp.wsgi

# flaskapp.wsgi
import sys
sys.path.insert(0. '/home/ubuntu/hoge') # wsgiは絶対パスによる参照が必要
from test import app as application # 後述

$ cd /home/ubuntu/hoge/test
$ vim __init__.py

# __init__.py
from flask import Flask
app = Flask(__name__)

@app.route('/')
def index():
    return 'hoge'

if __name__ == '__main__':
    app.run(host='0.0.0.0')

$ cd /etc/apache2/sites-available/
$ sudo cp 000-default.conf 000-default.conf.bk # バックアップ
$ sudo vim 000-default.conf

# 000-default.conf
LoadModule wsgi_module "/home/ubuntu/hoge/venv/lib/python3.8/site-packages/mod_wsgi/server/mod_wsgi-py38.cpython-38-x86_64-linux-gnu.so" # 補足①を参照
WSGIPythonHome "/home/ubuntu/hoge/venv"
WSGIScriptAlias /hoge /home/ubuntu/hoge/flaskapp.wsgi
<VirtualHost *:80>
        # The ServerName directive sets the request scheme, hostname and port that
        # the server uses to identify itself. This is used when creating
        # redirection URLs. In the context of virtual hosts, the ServerName
        # specifies what hostname must appear in the request's Host: header to
        # match this virtual host. For the default virtual host (this file) this
        # value is not decisive as it is used as a last resort host regardless.
        # However, you must set it for any further virtual host explicitly.
        #ServerName www.example.com

        ServerAdmin webmaster@localhost
        ServerName sample.com
        DocumentRoot /var/www/html

        <Directory /home/ubuntu/hoge > # アクセス許可のためのディレクティブ設定
                Require all granted
        </Directory>

        # Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
        # error, crit, alert, emerg.
        # It is also possible to configure the loglevel for particular
        # modules, e.g.
        #LogLevel info ssl:warn

        ErrorLog ${​​​​​APACHE_LOG_DIR}​​​​​​​​​​​​/error.log
        CustomLog ${​​​​​​​​​​​​​​​​​​​APACHE_LOG_DIR}​​​​​​​​​​​​​​​​​​​​​​​​​​/access.log combined

        # For most configuration files from conf-available/, which are
        # enabled or disabled at a global level, it is possible to
        # include a line for only one particular virtual host. For example the
        # following line enables the CGI configuration for this host only
        # after it has been globally disabled with "a2disconf".
        #Include conf-available/serve-cgi-bin.conf
</VirtualHost>

$ mod_wsgi module-config