In Drupal 8, pathauto patterns are configuration entities. You have at least two choices to provide them with your custom module:
(1) Provide your pattern as YAML configurations in your module's
config/install folder. They will be considered and installed to your Drupal, when your module will be installed. Easiest to create using the UI and doing a configuration export afterwards. Just remove the generated UUID of the pattern.
(2) On run-time or in your install/update hooks, you can use
Drupal\pathauto\Entity\PathautoPattern::create() or the \Drupal::entityTypeManager() to create them dynamically:
$data = [
'id' => 'my_pattern_machine_name',
'type' => 'canonical_entities:node',
'pattern' => 'my-path/[node:title]',
'weight' => 0,
$pattern = \Drupal::entityTypeManager()->getStorage('pathauto_pattern')->create($data);
You can then use the returned pattern entity in
$pattern to add selection conditions (e.g. bundles, language, ...) as required. Don't forget to save the entity afterwards, and - of course - check, if the pathauto module has been installed before or make it a dependency of your custom module.