The Service Cloud designation for a user is made by checking the Service Cloud User checkbox on the User record. This is referred to as a Feature License, which you can see the number of assigned and remaining users available under the Feature License section on the Company Information screen.

Once you get to the point in the process of selecting the users to share with during the Sharing Rule creation process, you will see 3 options in the Share with dropdown - Public Groups, Roles, and Roles and Subordinates. You can't create Sharing Rules to share with a Queue or individual users. Test this out by creating a Sharing Rule in your own organization.

Use Process Builder and Flow Builder to automate business processes that involve opportunity contact roles, so that your sales reps don’t have to do the work manually. For example, create a process that adds an opportunity contact role to each new opportunity that has a related contact.

Refer to this Spring '20 Release Note for details and an example - https://releasenotes.docs.salesforce.com/en-us/spring20/release-notes/rn_forcecom_flow_pbuilder_ocr.htm

The AppExchange contains applications built by companies that extend the Salesforce platform beyond its standard functionality. It is advantageous to leverage apps off of the AppExchange to save time compared to developing solutions on your own. The place to submit ideas to Salesforce is the Idea Exchange. Cases are submitted to Salesforce Support by clicking Help inside of Salesforce and selecting to contact support. The place to purchase additional Salesforce licenses for new users in called Checkout, or by calling your Salesforce representative.

History Tracking can be set on up to 20 fields per object. You can also track an additional 20 fields on Chatter, by the way (free tip). The Setup Audit Trail tracks who has been changed in your org from a customization standpoint and is not used for tracking changes to individual records. You can actually monitor users in the Debug Log for troubleshooting purposes, and it will reveal what records they are working on, but you have to guess which user, and the problem may be with a process or trigger that is causing these issues. You can also log in as another user and look at their Recently Viewed list, but this is also an inefficient approach. Stage is likely a field to already be tracked in most orgs, but I have been surprised by how many orgs have instituted no field history tracking. It's a quick win, when you are starting a new admin role to check for this.

Refer to https://help.salesforce.com/articleView?id=tracking_field_history.htm&type=0&language=en_US&release=206.13 for more details.

You can filter reports to such criteria as My Opportunities, which will display only opportunities that are owned by the current user. This is the ideal solution for this request. You could customize the existing report, or clone it to create a new report off of the existing, and then add the additional filter criteria. To set the org-wide default to Private would have far reaching implications beyond just this single report, and is a good example of not introducing unintended consequences in your org in the quest to quickly provide a solution. You also would not want to resort to code (Visualforce page), when a declarative option (clicks instead of code) is readily available to you.

This can be accomplished by creating a new custom Formula field. This Formula field would need a Return Type of Date specified. This would then display in Date format whatever the date would be 7 days prior to the entered Closed Date on an Opportunity. The actual formula would look like Reminder_Date__c = CloseDate - 7 - If you go through the process of creating this type of field in Salesforce, you'll actually see this exact formula as the example they give in the Formula Return Type selection for Date. Check it out.

The Process Builder has as an available action to post to Chatter. Refer to https://help.salesforce.com/articleView?id=process_action_post.htm&type=0 for more details.

A Roll-Up Summary field is a read-only field that displays the sum, minimum, or maximum value of a field in a related list or the record count of all records listed in a related list.

Custom Report Types define the set of records and fields available to a report based on the relationships between a primary object and its related objects. The other definitions are for Tabular, Summary and Matrix report format, which can be found here - https://help.salesforce.com/articleView?id=reports_changing_format.htm&type=0&language=en_US&release=206.13