If I install an app on Android outside Play Store. Will it ever be updated? How so?
If you app is not available in any store app you are using there will be no updates. If you want to update it you have to install a newer version of the app's APK file manually. If the app is available on Google Play or a different app store the situation is different:
On Android 7 and below there is only the Google PlayStore which can update an app. If the manually installed app is also available on PlayStore you can open it in the PlayStore App and install an available update. Once you have done this Google PlayStore will continue to include this app to the apps it will look for updates.
Other app store apps can also provide updates for apps, however the user has to manually confirm such updates (similar to a downloaded apk file that is installed).
Starting with Android 8 the user can configure which app is allowed to install other apps. Therefore alternativ appstores can provide an store app that can check installed apps for updates and install them accordingly.