Explanation
Freezing a user is a way to temporarily prevent them from logging in to Salesforce without deactivating their user record. This is useful when you need to perform some cleanup tasks before deactivating a user, such as removing them from being referenced in a hierarchy field like Direct Manager.
References:https://help.salesforce.com/s/articleView?id=sf.users_freeze.htm&type=5

Explanation
When you mark a field as required in Object Manager, the field is universally required to save a record on that object in the user interface. However, the field is not required to save records via the API on that object, unless you also mark it as required on the page layout.
References: https://help.salesforce.com/s/articleView?id=sf.fields_about_required_fields.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.admin_profile_picklists.htm&type=5

Explanation
Dependent picklist and global picklist are two options that can be used to meet this requirement. Dependent picklist allows users to create a conditional relationship between two picklist fields, where the available values in onefield depend on the value selected in another field. Global picklist allows users to create a set of picklist values that can be shared across multiple fields and objects, ensuring consistent values and reducing maintenance. References:
https://help.salesforce.com/s/articleView?id=sf.fields_about_dependent_picklists.htm&type=5https://help.salesfo