create a Modern team site without office 365 group and make externally shareable using c# csom code.

The modern site you are talking about in that case is the look and style of the modern page and modern libraries.

That does not require the Office Group capability, you can directly create the standard Teamsite (classical mode) and create a modern page into the sitepage library and set it as default page.

That is possible via CSOM code:

• https://docs.microsoft.com/en-us/sharepoint/dev/solution-guidance/modern-experience-customizations-provisioning-sites

Fab