EAS build creating apk instead of aab

I’m currently trying to create an aab file in order to upload it to my Google Play Dev account to create an internal release. I’m really confused though because every time I download the artifact after eas build completes, the file type is an apk and not an aab. I don’t have my eas.json file set up for the build to create an apk, and I thought the eas build command creates an aab by default. Someone please help me understand what it happening.