I am writing this quick tutorial in the hopes it helps someone else out there. There are a few guides out there to do similar tasks to this. They just are not quite what I wanted.

To give everyone an idea on the desired outcome, this is what I wanted to achieve:

Example user profile with 2 custom tabs in it.

Before I dive into this, I will mention that you can do this with views, if all that you want to produce is content supplied by views. Ivan wrote a nice article on this. In my situation, I wanted a completely custom route, controller and theme function. I wanted full control over the output.

Steps to add sub tabs

Step 1 - create a new module

If you don't already have a module to house this code, you will need one. These commands make use of Drupal console, so ensure you have this installed first.

drupal generate:module --module='Example module' --machine-name='example' --module-path='modules/custom' --description='My example module' --package='Custom' --core='8.x'

Step 2 - create a new controller

Now that you have a base module, you need a route

drupal generate:controller --module='example' --class='ExampleController' --routes='"title":"Content", "name":"example.user.contentlist", "method":"contentListUser", "path":"/user/{user}/content"'

Step 3 - alter your routes

In order to use magic autoloading, and also proper access control, you can alter your routes to look like this. This is covered in the official documentation.

# Content user tab.
example.user.contentlist:
  path: '/user/{user}/content'
  defaults:
    _controller: '\Drupal\example\Controller\ExampleController::contentListUser'
    _title: 'Content'
  requirements:
    _permission: 'access content'
    _entity_access: 'user.view'
    user: \d+
  options:
    parameters:
      user:
        type: entity:user

# Reports user tab.
example.user.reportList:
  path: '/user/{user}/reports'
  defaults:
    _controller: '\Drupal\example\Controller\ExampleController::reportListUser'
    _title: 'Reports'
  requirements:
    _permission: 'access content'
    _entity_access: 'user.view'
    user: \d+
  options:
    parameters:
      user:
        type: entity:user

This is the code that actually creates the tabs in the user profile. No Drupal console command for this unfortunately. The key part of this is defining base_route: entity.user.canonical.

example.user.content_task:
  title: 'Content'
  route_name: example.user.contentlist
  base_route: entity.user.canonical
  weight: 1

example.user.reports_task:
  title: 'Reports'
  route_name: example.user.reportList
  base_route: entity.user.canonical
  weight: 2

Step 5 - enable the module

Don't forget to actually turn on your custom module, nothing will work until then.

drush en example

Example module

The best (and simplest) example module I could find that demonstrates this is the Tracker module in Drupal core. The Tracker module adds a tab to the user profile.