Hi,
The architecture of microservices in App Engine is explained in this documentation and as a start you may migrate one of your existing monolithic applications to one with microservices by following this guide.
All your microservices can be run independently either in your local environment or on cloud, however the only way to execute code in these services is through an HTTP invocation, like a user request or a RESTful API call. Furthermore, autoscaling, load balancing, and machine instance types are all managed independently for services.