how i use requirements txt in virtual environment code example

Example 1: virtual env create python

#------FOR LINUX/MAC---------#
#installing venv 
sudo apt-get install python3.6-venv
#creating virtual env
python3 -m venv env
#activating virtual env
source env/bin/activate


#-------FOR WINDOWS----------#
#installing venv
py -m pip install --user virtualenv
#creating virtual env
py -m venv env
#activating virtual env
.\env\Scripts\activate

Example 2: python virtual environment

python3 -m venv env
python -m virtualenv env #py2

source env/bin/activate

#all this is on same directory

Example 3: how to use virtual environment python

python3 -m pip install --user virtualenv

Example 4: how to activate the pytho virtual environment

mypthon\Scripts\activate