Since you are bypassing the Google Play Store, you must allow installations from third-party sources.
After you've downloaded the APK:
You may have searched for "WhatsApp full" thinking it means a modded version with extra features (like hidden last seen, custom themes, or anti-revoke). Beware: whatsapp for android 442 uptodown full
The "full" from Uptodown simply means the complete APK (not a lite version). It does not mean a cracked or premium version – WhatsApp has no paid tier. Since you are bypassing the Google Play Store,