How to programmatically manage taxonomies? I'm working on a project with this requirement: there will be a couple of custom content types where many fields comes from taxonomies references.
Many of these taxonomies have plenty of terms (50+). These terms are periodically updated (added/corrected/removed) by a third party system in charge of this task, which later exposes these 'most current' terms list through a webservice.
So I don't need to force a human operator to manually check these terms lists and update D8 taxonomies accordingly, I thing that a good approach would be to periodically run a task to verify that webservice and then updated D8 with what have changed.
I've carefully read the following references:

Programmatically create term in Drupal 8?
Programmatically create a term?

Create taxonomy term programmatically on Drupal 8
http://www.drupal8.ovh/en/tutoriels/55/create-taxonomy-term-programmatically-on-drupal-8

Drupal 8 - Examples - create_taxonomy_term.php
https://gist.github.com/facine/35bb291811c146b6fc9e#file-create_taxonomy_term-php

Saving and retrieving taxonomy terms programmatically for Drupal 8
http://btmash.com/article/2016-04-26/saving-and-retrieving-taxonomy-terms-programmatically-drupal-8


The one that come closer to the project needs is the 4th one in the above list. It would suffice the requirements for now.
But I didn't quite get where this code should be typed. Does it need to be a custom module or should it go into themes/mytheme/mytheme.theme or so?
From where I stand, I think that an elegant solution is to have a routine somewhere (where?) that I could call via D8's cron so I can retrieve-then-update taxonomies periodically. But I have no clue on how I could achieve that goal.
Can someone provide any good references so I can fulfill this desire with code?
A simple way to set this up would be a cron hook.

function hook_cron


  Perform periodic actions.
  
  Modules that require some commands to be executed periodically can
  implement hook_cron(). The engine will then call the hook whenever a
  cron run happens, as defined by the administrator. Typical tasks
  managed by hook_cron() are database maintenance, backups,
  recalculation of settings or parameters, automated mailing, and
  retrieving remote data.
  
  Short-running or non-resource-intensive tasks can be executed directly
  in the hook_cron() implementation.
  
  Long-running tasks and tasks that could time out, such as retrieving
  remote data, sending email, and intensive file tasks, should use the
  queue API instead of executing the tasks directly. To do this, first
  define one or more queues via a \Drupal\Core\Annotation\QueueWorker
  plugin. Then, add items that need to be processed to the defined
  queues.


As mentioned a better way for D8 would be to use the queue API.

Or try the dev version of Feeds, which can do this without code:

https://www.drupal.org/project/feeds