First of all, make sure to register the component before initializing the root instance. Generally speaking, register the component first, and then create the vue instance. The instance should be created after the component is registered.
Registering global components with Vue.component
Create a vue instance
How can a component be used as a custom element in an instance template after it is registered
Register partial components with components
First declare a component template string
Create a vue instance
used in parent component
Here you will get the result
The global component <my></my> can be used normally in app1
The partial component <my-child></my-child> can only be used in app1