An application is not always available in the region or your device is not compatible. And, sometimes downloading a *.apk file can solve those problems. Here’s how to do it directly from Google Play and for free.
Websites to download APK
There are plenty of sites to grab the *.apk file from a Google Play link. The most recommended is the Evozi APK Downloader. From it, simply type the package name (with facebook.katana For example) or enter the URL of the app to download it. Then click the blue button and, when the page refreshes, the green button. With the *.apk file in hand, simply pass it to your phone with a USB cable, Google Drive or Telegram.
Other sites that do the same thing are APK Leecher and APK Downloaders. The first and second can even search by the application name. The second, unfortunately, does not accept the link: you need to write the package name.
How to install an APK
By default, Android is configured not to accept *.apk files downloaded by the user. To change this, go to Settings> Security. Below Device Administration, you will see Unknown Sources. Enable this option. Remember that this path may change on other devices, but just search for unknown sources in the Settings you should find the way.
After that, just download the APK from Google Drive, Telegram or install a File Manager to fetch the file from internal storage.