virtualenv - Birds Eye View of Understanding
virtualenv simply creates a new Python environment for your project. Think of it as another copy of Python that you have in your system. Virutual environment is helpful for development, especially if you will need different versions of the same libraries.
Answer to your first question is, yes, for each project that you use virtualenv, you need to activate it first. After activating, when you run python script, not just your project's scripts, but any python script, will use dependencies and configuration of the active Python environment.
Answer to the second question, pip freeze > requirements.txt will create requirements file in active folder, not in your project folder. So, let's say in your cmd/terminal you are in C:\Desktop, then the requirements file will be created there. If you're in C\Desktop\myproject folder, the file will be created there. Requirements file will contain the packages installed on active virtualenv.
Answer to 3rd question is related to second. Simply, you need to write full path of the second requirements file. So if you are in first project and want to install packages from second virtualenv, you run it like env2/bin/pip install -r /path/to/my/first/requirements.txt. If in your terminal you are in active folder that does not have requirements.txt file, then running pip install will give you an error. So, running the command does not know which requirements file you want to use, you specify it. 
I created a virtualenv 
C:\Users\admin\Documents\Enviorments>virtualenv django_1 Using base prefix 'c:\\users\\admin\\appdata\\local\\programs\\python\\python37-32' New python executable in C:\Users\admin\Documents\Enviorments\django_1\Scripts\python.exe Installing setuptools, pip, wheel...done. 
How do I activate it? source django_1/bin/activate doesn't work? 
I've tried: source C:\Users\admin\Documents\Enviorments\django_1/bin/activate Every time I get : 'source' is not recognized as an internal or external command, operable program or batch file.
Using Windows
Learning about virtualenv. Here is my understanding of it and a few question that I have. Please correct me if my understanding is incorrect. 
virtualenv are environments where your pip dependencies and its selected version are stored for a particular project. A folder is made for your project and inside there are the dependencies. 

I was told you would not want to save your .py scripts in side of virtual ENV, if that's the case how do I access the virtual env when I want to run that project? Open it up in the command line under source ENV/bin/activate then cd my way to where my script is stored?
By running pip freeze that creates a requirements.txt file in that project folder that is just a txt. copy of the dependencies of that virtual env? 
If I'm in a second virutalenv who do I import another virtualenv's requirements? I've been to the documentation but I still don't get it.
$ env1/bin/pip freeze > requirements.txt
$ env2/bin/pip install -r requirements.txt

Guess I'm confused on the "requirements" description. Isn't best practice to always call our requirements, requirements.txt? If that's the case how does env2 know I'm want env1 requirements? 
Thank you for any info or suggestions. Really appreciate the assistance.
I created a virtualenv  C:\Users\admin\Documents\Enviorments>virtualenv django_1 
Using base prefix'c:\\users\\admin\\appdata\\local\\programs\\python\\python37-32' 
New python executable in C:\Users\admin\Documents\Enviorments\django_1\Scripts\python.exe Installing setuptools, pip, wheel...done.
How do I activate it? source django_1/bin/activate doesn't work?
I've tried: source C:\Users\admin\Documents\Enviorments\django_1/bin/activate Every time I get : 'source' is not recognized as an internal or external command, operable program or batch file.