Relations between python 2.7 3 3.7, scipy, pip?

[snippsat]

And then I installed python3.7 through ppa, and I need to run it with

Look at pyenv Simple Python Version Management

Do I need to install pip for each version of python?
Do I need to install scipy, numpy for each version of python?

Yes to both.
Example see that both python and pip command point to 3.7.
Then will only install to 3.7.

# Check "python" command if point to python 2,try "python3"
tom@tom-VirtualBox:~$ python -V
Python 3.7.3

# Check "pip" command,if point to 2 "pip3"
tom@tom-VirtualBox:~$ pip -V
pip 19.0.3 from /home/tom/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip (python 3.7)

# Install scipy,see that numpy also automatic will be installed 
tom@tom-VirtualBox:~$ pip install scipy
Collecting scipy
  Downloading .......
Installing collected packages: numpy, scipy
Successfully installed numpy-1.16.3 scipy-1.2.1

# Test that it work
tom@tom-VirtualBox:~$ python
Python 3.7.3 (default, Apr 17 2019, 11:23:54) 
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import scipy
>>> scipy.__version__
'1.2.1'
>>> exit()