In TYPO3 9 is a new feature Site Management
, where a 'Site Configuration' can be created. Among the requested fields there is 'Entry point' where the domain name can be given which will be used for creating 'full URLs', allowing to renounce an extension like RealUrl.
After filling in the fields a file gets created at /config/sites/mysite/config.yaml
How can this file be loaded or change settings in it, depending on the so called application context / environment variable TYPO3_CONTEXT, since different domain names will be used for development, testing and production. Also locales could be spelled different.