This repository is holding all custom form fields.
Every form field is in it's own directory with instuctions to usage. Please see stucture of this repo.
To add a custom form fields, you should add your fields directory to include path.
For example:
JForm::addFieldPath(PATH_TO_MY_EXTENSION . '/fields');
For component, it usually means
JForm::addFieldPath(JPATH_COMPONENT . '/models/fields');