Most likely you would want to install the API Explorer into your local ACS installation. You can find the Alfresco API Explorer WAR file by searching for api-explorer on the Nexus Repository website available at https://nexus.alfresco.com/nexus/. Pick the version that closest matches your ACS installation.
To install the WAR file follow one of two approaches. If you are using a trial version of ACS then you follow the first approach described below. If you are using the Software Development Kits (SDK) you would want to follow the second approach (for more information, see Software Development Kits (SDK)). The main difference between the two is that the first one will lead to loss of the data that you are working with as the ACS trial Docker Compose file does not use volumes (i.e. it does not store data externally but instead inside the container).