Explanation
Lead assignment rule is a feature that can be used to route new leads to the correct sales person based on the lead address. Lead assignment rule can assign leads to users or queues based on certain criteria, such as lead source, industry, or location. Lead assignment rule can also send email notifications to the new lead owners or other recipients. References:
https://help.salesforce.com/s/articleView?id=sf.leads_assignment_rules.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