Let's say you have a custom module and you want to attach translation files to it. You want to import the translation files after installation or after you have updated the .po translation files.
Firstly make sure that the Interface Translation (locale) core module is installed. Use a folder named translations inside the module where the language files like de.po or fr.po can be found. To load the translations you have to insert the following lines into your example_module.info.yml:
'interface translation project': example_module
'interface translation server pattern': modules/custom/example_module/translations/%language.po
drush locale-check && drush locale-update && drush cr
To update existent translations you should take a look at the settings page (/admin/config/regional/translate/settings). You can use local translation files only or overwrite any existing translation.