We want to use a service account (SA) of a project to manage this project and other projects too.
Go to console.cloud.google.com/cloud-resource-manager to add projects we want, let's say aa-test-one, aa-test-two and aa-test-three.
On the page of aa-test-one, go to APIs & Services → Credentials → Create Credentials → Service Account → Give the neccessary information and grant this SA access to the project (eg. "Owner") → Done.
Copy the email of this SA, eg. admin@aa-test-one.iam.gserviceaccount.com.
Go to another project, let's say aa-test-two.
Navigate to IAM and Admin → IAM → Grant Access → Paste the email in previous step to the "New principals" & assign roles to this principal (eg. "Owner") → Save.