Python and multiple user access via super cool fancy website

[Aaron Christensen]

Hi all,

I am not sure if this is the correct venue for my question, but I'd like to
submit my question just in case.  I am not a programmer but I do have an
incredible interest in it, so please excuse my lack of understanding if my
question isn't very thorough.

As an example, a website backend is developed using Python.  Users can
submit their input through the website and PHP (or some other language)
transfers the user input from the website fields to a database such as
MySQL.  There is a main script called main_script.py which extracts the
user data from MySQL, processes it, stores output in MySQL and sends output
to the user (via webpage and email).

About main_script.py
# main_script.py extracts user input from MySQL, processes it, stores
output in MySQL and send output to user (via webpage and email).
# Inputs: User personal information such as age, dob, nationality, hobbies,
and 20 or 30 other fields
# Output: main_script.py is going to do something with it such as access
the database and some shelve files or other py scripts. I have no clue what
it's going to do, but my point is that the processing of the input to
output will take longer than simply a print('Hello, %r!' %user_name).

My question:  I am curious to know how Python handles something like this.
Let's say that there are 10, 20, 50, or even 1000 users accessing the
website.  They all put in their 20 to 30 pieces of input and are waiting on
some fancy magic output.  How exactly does that work?  Can multiple users
access the same script?  Does the Python programmer need to code in a
manner that supports this?  Are requests to the script handled serially or
in parallel?

I've tried some searches, but not getting much except for "appending to the
same file", etc.

I hope my question is a good question.  Thank you for your time!
Aaron