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Having been on both sides of the table...
As a vendor, I never shared policies outside of the organisation as they were considered to be proprietary. There were times when a policy included requirements that might lead to more questions regarding security operations, again not to be shared.
As a buyer, i had few vendors provide policies. Some would provide coverpages and tables of contents to give an idea of what was included in the policy.
If your vendor has an ISO27001 or PCI certification, or a SOC 2 report, policies are reviewed as part of completing these.