Both are Android package formats, and both end up on your phone as a file you tap to install. The difference is what each contains and who does the deciding.
An APK is a complete, ready-to-install application. It holds compiled code, resources, the manifest and the assets a given device is expected to need. APK was Google Play's original and long-standing format, and every sideloaded app you install is an APK. Because it is finished, what you download is exactly what runs.
An AAB is not something you install. It is what a developer uploads to Play — a container holding every module and asset variant the app has. Play then builds a tailored APK from that bundle for the specific device asking for it, stripping out resources that device will never use.
Delivery size. A single APK has to carry assets for every screen density, CPU architecture and language a device might have, most of which any one phone will never touch. Splitting delivery at install time removes a meaningful share of the download, often tens of percent on large apps and games.
.aab for direct download, it is not installable as-is and would need converting with a tool such as bundletool.Check the extension. .apk installs directly. .aab is a developer upload artifact for store distribution, and you need either the store or a bundle-to-APK conversion to use it.