I am converting my Android app from APK to AAB. However, when I uploaded the bundle.aab to AppCenter, the download was an APK? Shouldn't the download be a .AAB file?
The Android OS can only install APKs. If you upload Android App Bundles to the Google Play Store, the Play Store then generates optimized APKs for your end users.
Microsoft's App Center on the other hand generates an unoptimized universal APK.
"When you distribute Android Application Bundle (AAB), App Center generates a universal APK, signs it with a generated signing key, and distributes it to a device."
Source: https://learn.microsoft.com/en-us/appcenter/distribution/uploading