When you share something from your Workplace account, you can share it to a group or to your Timeline. When you share something to a group, your post may show up on your Timeline, depending on the privacy settings of the group.
If you share something with everyone at your company, anyone with a Workplace account at your company can see it. Nothing you share on Workplace is visible to people outside of your company.
Additionally, the admins who manage Workplace at the company can access anything you share from your Workplace account, just as they might access your work emails and other work files.