Explanation
A master-detail relationship is a parent-child relationship in which the master object controls certain behaviors of the detail object. The master object can be a standard or custom object, but not all standard objects support being a master. Roll-up summaries are fields that calculate the sum, count, min, or max of child records. The owner field on the detail records is not available and is automatically set to the owner of the master record.
References: https://www.forcetalks.com/blog/master-detail-relationship-in-salesforce/
https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.htm&type=5

Explanation
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

Explanation
Dynamic dashboards allow you to display data according to the security settings of the running user. If users have different access levels to data on dashboards, you can use dynamic dashboards to show them only what they are allowed to see.
References: https://help.salesforce.com/s/articleView?id=sf.dashboards_dynamic.htm&type=5

Explanation
Making a field universally required is a way to ensure that the field needs to be filled on every record; it prevents users from saving a record without entering a value in that field. It can be used to ensure that the billing system field needs to be filled on every invoice record by making it universally required in the field settings. Creating a process builder to set the field, defining an approval process for the child, or requiring the field on the record type are not ways to ensure that the field needs to be filled on every record; they either do not enforce data entry or only apply to certain scenarios or users. References:
https://help.salesforce.com/s/articleView?id=sf.customize_fields.htm&type=5

Explanation
Permission sets are used to grant additional permissions and access settings to individual users without changing their profiles or requiring a new profile to be created. You can use permission sets to extend users' functional access without changing their existing profiles. To meet the request of giving comprehensive edit access on Campaigns to a few marketing users who have read-only access by default, you need to create a permission set with edit access on Campaigns and assign it to those users. References:
https://help.salesforce.com/s/articleView?id=sf.perm_sets_overview.htm&type=5