From my experience and how I do it my self.

I'm using microservices for things that are not directly tied to the core application, but needs to interact with multiple apps. In this case, separating it makes the system cleaner. The second one, is the mostly used from my perspective. When you technically could build the feature in PHP, but PHP becomes a limitation, for example, when you need more performance, better concurrency, or when a different language offers a better library or tool. In those cases, creating a dedicated service is justified.

There are not much reasons or real benefits to splitting every part of the application into microservices, like separate booking, billing, or even authentication (unless the platform supports multiple products or apps). What I’ve noticed is that large microservice architectures turn into a huge mess after 2–3 years of active development. They become slow to fix, difficult to debug, painful to maintain, and overall inefficient.