how do you test microservices on their own, like not all together. do you have a plan for that

Hey everyone, I'm trying to get a better handle on testing in a microservices architecture. It's pretty easy to test services in isolation when they're new. But as dependencies grow, it feels like we end up doing a lot of integration testing just to get basic unit tests to pass. What are your go-to methods or tools for ensuring services can be tested reliably without needing the entire system up and running? I'm thinking about things like contract testing, mocks, stubs, or maybe something else entirely.