Migration of field_formatters from D7 to D9

Mapping field_formatters to migrate from D7 to D9

in Drupal 7 I've custom field_formatters. In Drupal 9 I've setup a custom module to define the corresponding formatters with the same names as the had before. Unfortunately I can't find any documentation to setup a migration yml-file to map the old formatters to the new once. Where can I find an example? Would appreciate a helpful hint.