Explanation
Contacts to multiple accounts is a feature thatallows you to associate a single contact with multiple accounts, both business and person accounts. This way, you can maintain the historical accuracy of the contact record without creating duplicate records for each account.
References:https://help.salesforce.com/s/articleView?id=sf.contacts_multiple_accounts.htm&type=5

Explanation
A criteria-based sharing rule is a tool that allows administrators to share records with certain users based on field values rather than ownership. For example, a criteria-based sharing rule can share all opportunities with a cross-sell opportunity type with a specific group of users. A public group is a collection of individual users, roles, roles and subordinates, or other groups that can be used to simplify sharing settings and other processes.
In this case, the administrator can add the subject matter experts to a public group and give them access to records with a criteria-based sharing rule that matches the cross-sell opportunity type. References:
https://help.salesforce.com/s/articleView?id=sf.sharing_criteria_rules.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.public_groups.htm&type=5

Explanation
A validation rule is a feature that allows administrators to define criteria for data entry or import operations and display an error message when those criteria are not met. For example, a validation rule can require users to select a product type or check a needs review box when saving an opportunity by using an OR function that evaluates both fields. If neither field is populated, then the validation rule will prevent users from saving records with an error message. References:
https://help.salesforce.com/s/articleView?id=sf.fields_about_validation_rules.htm&type=5