1. PYTHONPATH is an environment variable which you can set to add additional directories where python will look for modules and packages. e.g.:

   # make python look in the foo subdirectory of your home directory for   
   # modules and packages 
   export PYTHONPATH=${PYTHONPATH}:${HOME}/foo 
   

   Here I use the sh syntax. For other shells (e.g. csh,tcsh), the syntax would be slightly different. To make it permanent, set the variable in your shell's init file (usually ~/.bashrc).

2. Ubuntu comes with python already installed. There may be reasons for installing other (independent) python versions, but I've found that to be rarely necessary.

3. The folder where your modules live is dependent on PYTHONPATH and where the directories were set up when python was installed. For the most part, the installed stuff you shouldn't care about where it lives -- Python knows where it is and it can find the modules. Sort of like issuing the command ls -- where does ls live? /usr/bin? /bin? 99% of the time, you don't need to care -- Just use ls and be happy that it lives somewhere on your PATH so the shell can find it.

4. I'm not sure I understand the question. 3rd party modules usually come with install instructions. If you follow the instructions, python should be able to find the module and you shouldn't have to care about where it got installed.

5. Configure PYTHONPATH to include the directory where your module resides and python will be able to find your module.

Try appending to PYTHONPATH instead of overwriting it completely.

export PYTHONPATH=$PYTHONPATH:/home/dev/python-files

References:

According to the the Python documentation on PYTHONPATH

Augment the default search path for module files. [...]

The default search path is installation dependent, but generally begins with prefix/lib/pythonversion (see PYTHONHOME above). It is always appended to PYTHONPATH.

meaning that some values exist in PYTHONPATH and the default search path is also only appended.

Additionally, this blog post (Archive.org link) also explains clearly why you need to append to PYTHONPATH and not overwrite it. Scrolling down to the section - Special cases and examining the search path explains it clearly (unfortunately no relative URL to that link so you'll have to scroll). Although the user gives the examples on a mac they are very much relevant to any platform

I'm assuming that when you installed anaconda 2, you manually set the PYTHONPATH environment variable, by putting something like

PYTHONPATH=/home/user/anaconda2/python
export PYTHONPATH

in your .bash_profile or .bash_rc.

But since you deleted the /home/user/anacanda2/ directory, that path no longer exists.

Thus you want to change PYTHONPATH to point to the executable in /usr/lib, by changing the above to

PYTHONPATH=/usr/lib/my_python_distribution
export PYTHON

If you're using bash (on a Mac or GNU/Linux distro), add this to your ~/.bashrc

export PYTHONPATH="${PYTHONPATH}:/my/other/path"

Use single-quotes:

$ echo 'export PYTHONPATH=$PYTHONPATH:/path/to/new/python/module' >> .bashrc
$ cat .bashrc 
export PYTHONPATH=$PYTHONPATH:/path/to/new/python/module

The shell does not perform variable expansion on single-quoted strings.

Note also that, if you are writing to ~/.bashrc, you should not need sudo. A user should own his own ~/.bashrc. Further, as written, the sudo command only operated on echo. The redirection >~/.bashrc is done with the user's level of permission. Since echo has no need of and gets no benefit from sudo, sudo is a practically a no-op. [Hat tip: tripleee]

I'm running Mint 19.3 and kernel version 4.15.0-106-generic - so not far off from you.

(python3-venv) me@mycomp:chap10-Batteries_Included$ cat/etc/*release
DISTRIB_ID=LinuxMint
DISTRIB_RELEASE=19.3
DISTRIB_CODENAME=tricia
DISTRIB_DESCRIPTION="Linux Mint 19.3 Tricia"
...
(python3-venv) me@mycomp:chap10-Batteries_Included$ uname -r
4.15.0-106-generic

running in a Python 3 virtual environment. I'm working on "Beginning Python, From Novice to Professional" by Magnus Lie Hetland and got to Chapter 10 where it talks about this very thing. When I first get into my shell, I find nothing assigned to $PYTHONPATH either:

(python3-venv) me@mycomp:chap10-Batteries_Included$ echo $PYTHONPATH
 
(python3-venv) me@mycomp:chap10-Batteries_Included$

So I do what is suggested everywhere on the web, and in the book($PWD being my working directory, which happened to be the directory I wanted added):

(python3-venv) me@mycomp:chap10-Batteries_Included$ export PYTHONPATH=$PYTHONPATH:$PWD
(python3-venv) me@mycomp:chap10-Batteries_Included$ echo $PYTHONPATH
:/home/me/Documents/CodingProjects/python/Beginning-Python/book/chap10-Batteries_Included
(python3-venv) me@mycomp:chap10-Batteries_Included$

And there we go, it's there:

(python3-venv) me@mycomp:chap10-Batteries_Included$ python -O
Python 3.6.9 (default, Apr 18 2020, 01:56:04) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys, pprint
>>> pprint.pprint(sys.path)
['',
 '/home/me/Documents/CodingProjects/python/Beginning-Python/book/chap10-Batteries_Included',
 '/usr/lib/python36.zip',
 '/usr/lib/python3.6',
 '/usr/lib/python3.6/lib-dynload',
 '/home/me/python3-venv/lib/python3.6/site-packages']
>>> 

I noticed, and would have commented had I been able to without having 50 points on here, that you have export export PYTHONPATH=$PYTHONPATH:.... - you only need export once. You can add this export PYTHONPATH=... to your ~/.profile to make it permanent, too. I hope this helps.