Mobile projects currently build for Android only, iOS isn't available yet. From your project's Build section, you get a signed, installable .apk file by default.
What "sideloadable" means
This .apk isn't published to the Google Play Store, there's no store listing, review process, or store-managed updates. Installing it means enabling "install from unknown sources" on the Android device and installing the file directly, this is normal for a self-distributed app, but is an extra step compared to a Play Store install, worth mentioning to anyone you send it to.
Distributing it
Share the .apk file however suits you, direct download link, file transfer, and similar. Since it's signed consistently, a device that already has an earlier version installed can install a newer .apk over it as an update, without needing to uninstall first.
Building an .aab instead
If you actually want to publish to the Google Play Store, build an .aab bundle instead, the format Play has required for new uploads since 2021. The Build section lets you choose either format. An .aab can't be installed directly on a device the way an .apk can, it only ever makes sense as a Play Store upload, use .apk for sideloading and sharing it directly.