Question about a microservice based app

I am a Jr devops engineer at the moment, about 3 months in. I do a lot of work with containerization (mainly setting up testing environments and CI/CD pipelines), Linux, and IAC

I’m developing a web app to expand my developer/DSO skills and I have a couple questions, I want it to be microservice based (e.g. a docker container runs a chat app, the container is orchestrated with K8s for redundancy). I hear a lot about apps converting their infrastructure from monolithic to microservice but I’m not quite sure how this would work. Would I simply embed the container within an iframe in the app? Do I need to make an API? Is my understanding of microservices incorrect?…all help appreciated immensely