Ok. But i have a site in www.domain.tld. An webapp in www.domain.tld/myapp And i want install other app (not developed for me, an app with own self enviroment and files) on www.domain.tld/other_app
That should not be a problem I think, although I've never tried it.
Install the index.php and the .htaccess file in your other_app folder, and make sure that index.php has the correct APPPATH, COREPATH etc set. Two apps can not share an app folder, but they can share everything else (core, packages, modules, composer vendor).
The only possible issue might be if "domain.tld/other_app" is requested, so without a trailing slash. This will probably picked up by your main app, instead of loading the root page of your other_app. If that is the case, you need to add a rewrite rule to your main app's .htaccess file to make sure it's rewritten to "domain.tld/other_app/" or "domain.tld/other_app/index.php". You'll have to try to see what works.