How can a Loan record be shared with a specific Underwriter on a loan-by-loan basis?

The option involving creating a lookup relationship from the Loan object to the User object and using a trigger for sharing is a valid approach for sharing Loan records with specific Underwriters on a loan-by-loan basis. When a lookup relationship is established, it enables each Loan record to relate directly to a User (in this case, the Underwriter).

The trigger would allow the system to programmatically share the Loan record with the designated Underwriter based on specific business logic. For example, when a Loan record is created or updated, the trigger can evaluate conditions and determine whether to grant access to the associated Underwriter. This method provides precise control over individual records and ensures that only the relevant Underwriter has visibility into a particular Loan record without affecting others.

While criteria-based sharing rules offer a broader approach to sharing rules across multiple records based on certain criteria, they may not be effective for on-the-fly, loan-specific sharing scenarios. Similarly, a master-detail relationship would create tighter coupling between records but is more suited for scenarios requiring ownership and cascading rules rather than individualized sharing. Lastly, manual sharing allows a user to share records but is labor-intensive and not scalable for handling many Loan records or dynamic relationships.