If you are using an app or game that is not a Play store then it must illegal app in this case you can use alternatives to the Play store but it is useful in few scenarios.
Many apps provide an Update option inside their app you just have to check settings.
To update such apps you should remember or Bookmark the website so that whenever updates come you can download the latest version of it.
If you don’t remember that site and still want to update an app that is not linked to Play Store then firstly check out the latest version of that specific app and go to Google and search for that app with App name + what version you want + specific version of that game like mod or anything else, Now go to different sites you will definitely find the latest version of your app for sure.
Have any question so don’t hesitate at all assume me like your friend and ask your question in comment box present below.