Modules (extensions) are necessary tools because the core of Drupal actually does not contain enough resources to make a nice and safe site. Compare it to a car where only one wheel, gas and brake are provided. No heating, GPS, cruise control, ...
Some modules are already in the core but are not yet activated. So you see that the 'responsive image' module (creates a responsive graphic style) is already installed. You just turn on the check mark. The phone module is also one of those.
At the time of this writing, 7697 modules for Drupal 8 are available. On this site I have made a selection and discussed 450 of them at present.
On drupal.org you can see the modules...
You filter on the Drupal version 8.
You can now go to a particular module using the search box on drupal.org, but is easier to search for the module in Google. Search by drupal.org is not so convenient.
If you find the module you see, go to the bottom of the downloads, ( and their indication of stability). Take green if possible, but I always take the latest version to see what the possibilities are ...
You can now download the module in several ways. Right-click on a zip or tar.gz link (both are possible) and copy the link address.
On your website you are going to install a new module.
You paste the link in the upper field. The other option is to install a downloaded module. The first way is faster, of course. In very rare cases, there are modules that are not available through drupal.org (GitHub). Than you should install your file on this way.
After installing (= actually download only), you should go to the module list and enable it. Modules sometimes offer different possibilities. You check just what you need. What you need to check is always in the discussion of the module. This is different from module to module. The English drupal.org explanation is rather brief .. Drupal designers are not great orators LOL ... .
In some cases, you can not check the module because that depends on another module .. So Focal Point depends on the Crop API module and is not in the core. So you must also download it first, otherwise you can not check the main module.
You see only what you need to install even if you click on the arrow! Well hidden if you ask me ...
In some cases, there is no tar or zip. Below is a link to a dev (development version). You can not copy the link. Do not choose a dev version if there is an alternative.
By clicking the link you'll come up on the proper link.
In some cases, you need a third party. A third party is another upload of folders and files that you can not install with the above method.
The Roundabout slider i take as an example: https://www.drupal.org/project/roundabout
In the manual of the module you see where get your supplies. These files and folders you should download and upload them to the server in the correct folder. You do this with FTP programs like FileZilla or directly in the control panel of your hosting. The CP (Control Panel) has the advantage that you can upload the zip or tar and unpack it on the server. Useful.
The directory is called libraries. If it does not exist you have to create the folder on your server. The folder is in the root (eg next to the modules folder). Below I have other third parties .. The syntax of the folders is very important and case sensitive. Reference is made by the module to this third party folders and for this it has to be right naturally.
Professionals install modules with Linux commands. You should therefore have SSH access to your site. A good hosting has. Moreover .. Drupal 8 can not run without this SSH as some modules can only be installed with composer.
You can now use Putty (additional program you install) or via the Terminal entering your commands.
Via the command cd (change directory), you can navigate ls (list of files and folders) and see where you are on your hosting. Choose the correct folder if you have subdomains ..
Then we use drush. This is already running on a good hosting. We are going to install Focal Point. (Machine name = focal_point)
drush and focal_point -y Note that drush not just download the module, but also the dependent modules (Crop) and he activates them.
You have sometimes to look if everything is checked. If several sub-modules come along with installation, drush will only check the main module.
Drush is no different thing than a quick way to install the modules just as you would have it installed manually. Drush can still be used for many other things like updates etc .. Here we are talking only about the installation of modules. You can also install multiple modules. Handy if you will have a list of your favorite modules in notebook. Copy and paste and enjoy ..
In the same window you can also use composer. On a good hosting, this is already installed, otherwise you have to install it yet. There's a discussion for it on this site.
Some modules can only be installed with composer because they also make changes in the vendor directory. Composer is recommended for Drupal 8 as the best way to install modules.
Oddly enough you can activate no modules with composer. You have to do it manually or via drush. Composer does hold a file whit what you have installed. That way you have made a list of things (in composer.json) and you can also perform the updates there. It is intended that composer also install third party applications. This is not always so on this moment. You can add it manually to your compser.json file.
For example, I install the handsome Leaflet module. An alternative to Google maps module as it has become paying.
composer require drupal / leaflet
As said, you should still check yourself what you need. Leaflet was dependent on the Geofield module. Which was installed at the same time, along with many other things as you can see.
The file composer.json says leaflet was installed:
Know that when upgrading the core of Drupal, the composer.json file is overwritten. Before you do an upgrade you must make a copy of this file to put it back after the update ...