Drupal 8.x

I'm a complete Drupal newbie trying to install Drupal 8 for the first time on a new server with Ubuntu 18.04, Apache 2.4, MariaDB 10.1 and php 7.2. I downloaded the software using composer as indicated in section 3.5 of the online User Guide.  Next,in section 3.7, it says to go to the URL I set up for the site, where I should find an installation page. When I do this, I get an index of the directory with these files and directories in it:

I'm really stuck and would appreciate some help! 

I have two custom content types and forms to create them (so the user can just fill the form to create a node, which is also a taxonomy term).

For example - I have a custom content type Person - and user can fill name, birth date, occupation, etc. After saving it becomes a taxonomy term of Persons dictionary.

The same with Project - Projects dictionary.

The problem is that I need to add the ability for the user to fill info about projects inside a Person form and vice versa.

When logged in user is coming on the website where maintenance mode is switched on he is logged out. I know this behaviour is intended and stored in Drupal\user\EventSubscriber\MaintenanceModeSubscriber , my question is - why is it so?

Why need to logout user that is visiting website in maintenance mode.

Hello friends,

I recently purchased a theme for my Drupal website. Inside the theme folder there is -> templates/page--front.html.twig & page.html.twig

The a snippet of the <footer></footer> in page.html.twig looks as follows: 

Hello friends,

I have a home.html.twig page stored in the /templates folder of my custom module.

How would I go about setting this page to be the front page of my drupal site?

It should probably also be noted that I am using a theme that I have purchased.

Any advice would be GREATLY appreciated <3

Hello,

Hope everyone is doing well...

I am planning to build a matrimonial website...

I want to add custom radio buttons, custom check boxes, custom select options to the standard registration form...

Are there any Drupal 8 plugins or modules to accomplish this?...

Is it possible to add custom fields without any Drupal plugins or modules?...

Please provide some help or idea to me...

Thank you very much...