i would like to know how can i run Validations/Actions (like field validations) after i select an item from a lookup list.
To set validator on some field, usually setValidationConfig method is used.
Validation method, that is set for some field, triggers each time the value in this field changes.
Article about it: https://academy.bpmonline.com/documents/technic-sdk/7-13/how-add-field-validation
If the task is to trigger custom validation methods on other fields it is achievable via dependencies. Please check the example of usage: https://community.bpmonline.com/questions/handler-dependant-detail-update