What is the difference between "Add user group" in "Organizations REST API" and "Jira Cloud platform REST API"?

I’m trying to create a group with REST API and add users to it.
After investigating the REST API, ‘Create group’ and ‘Add user group’ existed in both ‘Organizations REST API’ and ‘Jira Cloud platform REST API’. What is the difference between the two REST API?

I thought that if I created a group using the “Jira Cloud platform REST API”, it could only be referenced from that site.
However, I was able to reference the group from sites other than those created using the REST API.
In that case, you won’t be able to tell the difference between the two. Is it the same?

Thanks for your help.