Translation tools help you by suggestions based on base translation.
Translation memory searches for similar texts in your project. Each suggestion is based on different key in the project and the percentage shows similarity between base translations.
Machine translation use external providers to translate base translation to currently edited language. Number of machine translations that you can use is limited by credits which are shown in language settings.
To enable machine translation when self hosting check out configuration.
Automatic translation of new keys
Automatic translation is applied when base translation is created and it tries to translate to other languages currently in the project, which are not already filled. It's disabled by default, you can enable it in language settings. Translations that were translated automatically (and not edited by human) are marked by provider icon or translation memory icon.