For Android, the apk's can be installed to any device, simply enable sharing on your app settings and then send them the share link (see image below).
For iOS, to distribute development versions of your app outside the app store, you do need to include all test device UUID's in your App Mobile Provisioning Profile. Its a limitation created by Apple. Once that is done, you can again send the share link out and users can install directly.
Thanks for your answer.
As I researched it is the ad hoc provisioning profile.
For anyone who wants to get further infos :Exporting Your App for Testing (iOS, tvOS, watchOS)
There some more ways to distribute not via app store, but for me this seems to be the best match.