Explanation
One of the possible causes for being unable to add a new user in Salesforce is that the username is already in use by another organization. Usernames must be globally unique across all Salesforce orgs, so if another user has claimed that username before, it cannot be used again. To fix this issue, choose a different username that is not taken by anyone else. The username is not restricted to a domain specific to my domain unless specified by an administrator. The email address used for the username does not have to match a contact record. The email used for the username can be any valid email address. References:
https://help.salesforce.com/s/articleView?id=sf.admin_usermgmt_add.htm&type=5

Explanation
Outbound message allows you to send information to an external system as part of a workflow rule or approval process. You can use outbound message to specify which fields to send and which endpoint URL to send them to.
References: https://help.salesforce.com/s/articleView?id=sf.workflow_om_considerations.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.workflow_om_define.htm&type=5

Explanation
To require the Amount field during the negotiation stage for sales users but allow their managers to make changes, the administrator should configure a validation rule that checks if the user profile is not a sales manager, the stage is negotiation, and the amount is blank. This will prevent sales users from saving the record without entering an amount, but allow sales managers to do so. Making the field required for all users will not meet the requirement. Creating a formula field or assigning the administrator profile to the managers will not affect field requirement. References:
https://help.salesforce.com/s/articleView?id=sf.validation_rules_overview.htm&type=5

Explanation
A roll-up summary field is a type of field that calculates values from related records, such as the count of child records or the sum of a field on child records. In this case, the administrator can add a roll-up summary field to the open house object to track the number of visitors by counting the child records on the visitors object.
References:
https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.htm&type=5

Explanation
Assignment rules are tools that allow administrators to automatically route records to users or queues based on certain criteria. For example, an assignment rule can assign cases to different queues based on case priority, origin, type, or other fields. Assignment rules can be triggered when records are created manually, via email, web, or API. Assignment rules consist of multiple rule entries that define the criteria and actions for each assignment scenario. References:
https://help.salesforce.com/s/articleView?id=sf.customize_leadrules.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.customize_casesupport_assign.htm&type=5