When using out-of-the-box Broadleaf Microservices flows, the components are already aware of the different microservices they are extending. So if you extend and override a Cart component via Spring bean, then the overflow uses that bean when it gets to CartServices instead of the framework bean.
If this is a new custom flow you are looking to implement, you will need to configure cross-microservices communication, as described above. Depending on your need, there might already be an out-of-the-box message producer that you can listen to. There’s further documentation on configuring a message listener here: Broadleaf Dev Central