[SalesForce] How to assign a sharing rule to a specific user only

Imagine you have two users. Both are in the same profile, which allows them to Create, Edit object XYZ. But these two users are in different parts of the role hierarchy.

If User A Creates an XYZ, can user B edit it? The answer is only if the record is shared with him. If the Org Wide Default is Private then he will not even be able to see the record that User A has created (unless it is explicitly shared with him). He can still create his own XYZ records, and edit those. Likewise if the Org Wide Default is public read only, User B could see it, but not edit it despite what his profile allows. He can edit only his own records or those shared with him.

Sharing also works up the role hierarchy. So User A's boss can see the XYZ record that User A created. If his profile allows, he can edit it, but his profile may not.

You can think of the profile as "What can a user do with the XYZ object" and the sharing as "Which particular XYZ records can he or she do it with"

Does this help clarify?

EDIT--- Answering your additional questions

1. If OWD is Public Read / Write but the profile does not have access to Read or to Edit then you cannot Read or Edit.

2. If OWD is Public Read / Write on object XYZ and profile has Read permission but not Edit, then the user with that profile can only Read XYZ (all of them)

3. If a user doesn't even have Read access to object XYZ then they cannot see any object XYZ regardless of the Org Wide Default.

4. Not correct - the owner who is higher on the role hierarchy has sharing access to all the XYZ that his subordinates own. However, if the higher user has a profile which doesn't include read or edit then he cannot read or edit those records.

The record owner is automatically granted Full Access, allowing them to view, edit, transfer, share, and delete the record

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_bulk_sharing_understanding.htm

If the user is the owner of the record then they have implicit visibility. You will need to remove that user from being the owner.

Opportunities have an additional setting under roles that provides access based on account ownership. Make sure the users role is set to "cannot access opportunities that they do not own" so if they do not own the app but own the related account they cannot see it

Optionally you can override the View to use a VF page with an <apex:detail> tag and if the owner should not see it then present a message instead of the <apex:detail>