To require users to choose a specific search engine from a picklist when lead source is "Search Engine", and hide it otherwise, the administrator should create a picklist for specific search engine on the same screen as lead source, and set conditional visibility so that it is only shown when lead source is "Search Engine". This will make sure that users see only relevant fields based on their input. A decision element will create an extra screen that may disrupt user experience. An assignment element will not affect field visibility. A validation rule will not hide fields but only show errors when values are invalid. References:
https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_screen_components_picklist.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_screen_components_conditional_visibility.ht