What's the build order of product-am and carbon-apimgt?

First you need to build the carbon-apimgt repo and next, build the product-apim repo. Then the product pack can be found within the product-apim/modules/distribution/product/target directory. You can get the relevant zip file of the pack and run it by following the steps mentioned here.

If you are building the master branch, make sure you have fetched the latest changes from both the carbon-apimgt repo and the product-apim repo. First build the carbon-apimgt master branch. Then copy the value within the <version> tag of the pom.xml of the carbon-apimgt master branch and add it as the <carbon.apimgt.version> of the pom.xml of the product-apim master branch. (The value within the <version> tag of carbon-apimgt pom.xml and the value within the <carbon.apimgt.version> tag of the product-apim pom.xml should be the same). Then build the product-apim master branch. Then the product pack can be found within the product-apim/modules/distribution/product/target directory.

If you want to build a specific version of API Manager (Let's say API-M 4.0.0), first checkout to the relevant tag of the released version in the product-apim repo (v4.0.0). Then check the <carbon.apimgt.version> tag of the product-apim pom.xml (9.0.174). Next, go the carbon-apimgt repo and checkout to this relevant tag (v9.0.174). Then you can first build the carbon-apimgt repo and afterwards build the product-apim repo to get the relevant API-M pack.