To show different picklist values for sales and marketing users, you can either use record types or profiles.
Record types allow you to have different picklist values for different page layouts assigned to different profiles. Profiles allow you to restrict picklist values based on user permissions.
References: https://help.salesforce.com/s/articleView?id=sf.customize_recordtype.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.admin_profile_picklists.htm&type=5

Flow Builder allows you to create an automated business process that can update records based on certain criteria. You can use a scheduled flow to run once a week and count the number of open cases related to an account.
References: https://help.salesforce.com/s/articleView?id=sf.flow_builder_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.flow_concepts_scheduled_start.htm&type=5

Role hierarchy and sharing rules are two features that should be used to open up access to Opportunity records for sales users working on collaborative deals. Role hierarchy can be used to grant access to records owned by or shared with users who are below them in the hierarchy. Sharing rules can be used to extend sharing access to users in public groups, roles, or territories based on certain criteria, such as record owner or field value.
References: https://help.salesforce.com/s/articleView?id=sf.admin_sharing.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.security_sharing_rules.htm&type=5

A lookup field is a type of field that allows administrators to create a relationship between two objects by linking records from one object to another object. For example, a lookup field can link an account record to a purchase record by storing the account ID on the purchase record. A lookup field allows users to select an existing record from a pop-up window or create a new record from the same window. References:
https://help.salesforce.com/s/articleView?id=sf.relationships_lookup.htm&type=5

Splits is a feature that should be used to meet this requirement. Splits allows users to assign credit to opportunity team members based on the level of effort contributed by each person toward each deal. Users can create different types of splits, such as revenue or overlay splits, and specify the percentage or amount of credit for each team member. References:
https://help.salesforce.com/s/articleView?id=sf.forecasts3_splits_overview.htm&type=5