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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.

Monorepos makes the dev experience simpler, at the expense of more complex CI

I was resistant at first, but have found it to be a worthwhile trade-off

By only testing and building packages that has a dependency on the changed package.

And how do you determine that dependency structure easily? What tooling helps with this?