Explanation
Discount is not a standard field on the Opportunity object, so you need to create a custom field to capture the discount amount or percentage for each opportunity. To set up an approval process where opportunity discounts over 30% need to be approved by the VP of sales, and any discounts above 10% need to be approved by the user's manager, you need to configure two separate approval processes with different entry criteria based on the discount field value and different approvers based on their roles. References:
https://help.salesforce.com/s/articleView?id=sf.approvals_getting_started.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 sharingrule that matches the cross-sell opportunity type. References:
https://help.salesforce.com/s/articleView?id=sf.sharing_criteria_rules.htm&type=5https://help.salesforce.com/s/a