Because this is a fan-made project and a modified file, you will not find it on the official Google Play Store. You must install it manually via "sideloading."
This specific APK is a Modded APK created by Remo_Wind. The "attack on survey corps 0.21.3 android mod" likely provides enhancements based on common mod trends for this title. Attack On Survey Corps 0.21.3 Android -MOD-.apk
Downloading modified files from aggregate mirror sites deprives the original independent developers of traffic, support, and feedback on their official development channels. How to Safely Access the Game Because this is a fan-made project and a