Account Sharing URL:
https://YourDomin.my.salesforce.com/p/share/AccSharingDetail?parentId=IdOfAccount

Case Sharing URL:
https://YourDomain.salesforce.com/p/share/CaseSharingDetail?parentId=IdofCase

Lead Sharing URL:
https://YourDomain.salesforce.com/p/share/LeadSharingDetail?parentId=IdOfLead

Opportunity Sharing URL:
https://YourDomain.salesforce.com/p/share/OppSharingDetail?parentId=0065w000025ZJXy

Custom Object Sharing URL:
https://YourDomain.salesforce.com/p/share/CustomObjectSharingDetail?parentId=a065w00000iSO8a

Example for Custom Object Sharing URL:

To learn more about sharing, please check – https://www.infallibletechie.com/2015/12/sharing-button-in-salesforce.html