I have tried to customize robots.txt using domain and robotstxt module without susceed. My use case is specific to pantheon that blocks drupal robots.txt by default in order to remove multidevs testing environments from crawl.
On a pantheon Drupal 10.3 installation install robotstxt and domain modules using composer. Configure your domains and enable robots using per domain configuration on /admin/config/search/robotstxt
Needs work
1.0
Code
You should use only one of this module or RobotsTxt module because both modules have the same path /robots.txt. If you have different domains and you want to have different "robots.txt" content, you can install this module and configure from the following path /admin/config/domain/robots_txt/{domain}.