In which two ways can Opportunities with a Community User be shared?

Sharingsets allow you to grant access to records that have a lookup relationship to the same account or contact that the community user has. You can use sharing sets to share opportunities with partner community users who belong to the same account as the opportunity owner.

Criteria-based sharing rules allow you to share records based on field values. You can use criteria-based sharing rules to share opportunities with partner community users who meet certain criteria, such as region, industry, or status.