Hi @Carl Williams ,
According to my research and testing, the top link bar of SharePoint displays the subsite by default. As long as you checked these two when creating the subsite:
If you want to connect back to the parent site from the subsite, you can add the url of the parent site to the quick launch:
Updated on 5/2/2023
You can manually add the url of the parent site to it by editing the Top link bar.
Go to Site Settings, click the Top link bar under the Look and Feel section to enter the following interface:
Click Stop Inheriting Links, then click New Navigation Link. Fill in the url of the parent site in the following interface, and click OK.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.