[Answered ]-DRF, why my root api urls are got mixed (combined)?

Try to specify the basename to the routes like this:

# urls.py
router = routers.DefaultRouter()
router.register(r'users', UserViewSet, basename="users")
router.register(r'user-activity', UserActivityViewSet, basename="user-activity")

urlpatterns = [
    path("", include(router.urls)),
]

PS.: Some DRF versions call that as base_name