Answers for "how to register template folder in settings.py django 3.1.2"

dirs' base_dir / 'templates' error

Incorrect syntax in polls app tutorial by Django

Use this: 
os.path.join(BASE_DIR, 'templates')

instead of this:
BASE_DIR / 'templates'

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        'DIRS': [os.path.join(BASE_DIR, 'templates')],
        'APP_DIRS': True,
        'OPTIONS': {
            'context_processors': [
                # codes...
            ],
        },
    },
]