You use a build system. There are a number of open source ones, we use Bazel. In a nutshell, it keeps track of dependency graphs, so you can test only affected

You use a build system. There are a number of open source ones, we use Bazel. In a nutshell, it keeps track of dependency graphs, so you can test only affected projects. It can also cache execution of unchanged transitive steps, so you can skip builds/tests that were already run previously (e.g. you could skip most of a large 2nd CI run if all you did in a code review was edit one file. You can also parallelize execution across cloud nodes.