Dear Gary Speirs,
Good day!!
As per the description shared, I understand your concern i.e., you have signed up for the SharePoint Online standalone subscription i.e., SharePoint Online Plan 1 or SharePoint Online Plan 2 and looking to download the SharePoint software on your computer.
If my understanding is correct, I would like to convey that SharePoint Online is a cloud based service where there is no any application to use SharePoint Services either on the Windows or Mac, where we just require the browser to access the SharePoint Online services. (SharePoint application is available either on Android OS or iOS, where it's free to install either on Play store or App Store)
Open your browser on your Mac device> Access https://portal.office.com> Enter the Office 365 accounts which you have registered for SharePoint Online license (it will be like ******@companyname.onmicrosoft.com)> Click on the 9 dots at the top left> Click on the SharePoint> Start using the SharePoint Online services.
Here are the references for SharePoint Online guide:
Get started with Microsoft SharePoint Online (User training) – Microsoft Adoption
SharePoint video training - Microsoft Support
If there is any misunderstanding or confusion, we apologize and request you to share the more detailed information with some screenshots which will help us to better understand and guide you further.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia