Short answer
In Marketing > Social, create a post, then click Review & Schedule to check content, captions, campaign and timing for each account. Publish immediately, pick a time, or choose from your saved publishing times. Approvals route posts to an approver before publishing; approvers can act from the mobile app too.
1. The social tool
My demo portal has no social accounts connected, so the tool shows its intro.
2. Approvals
Content approvals are covered in HubSpot content approvals.